Add well-named snapshot update modes, at measurement start do only-invalid snapshot, a few driver fixes of snapshotting

[astafan8]

Reworks the update argument of snapshot / snapshot_base (across the whole
Metadatable hierarchy) into an enumeration and refreshes only invalid caches by
default.

• New values: "All" / "Only_invalid" / "Never". Legacy True / None /
  False still work as deprecated aliases (static-only via @deprecated, no
  runtime warning).
• Default is now "Only_invalid" for snapshot / snapshot_base /
  print_readable_snapshot (was effectively "Never"), so .snapshot() now
  refreshes parameters with an invalid cache. Pass update="Never" for the old
  behavior. See 8354.breaking.
• Measurement: station snapshot uses "Only_invalid"; registered-parameter
  snapshots pinned to "Never".
• New public qcodes.metadatable.normalize_snapshot_update + SnapshotUpdate type.
• Docstrings, the "Working with snapshots" notebook, driver examples and tests use
  the enum. Newsfragments: 8354.new, 8354.breaking.

Validation: pyright (src+tests) and mypy -p qcodes clean, pre-commit passes,
full suite 3135 passed / 268 skipped.

[codecov]

Codecov Report

❌ Patch coverage is 70.90909% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.09%. Comparing base (ce48db5) to head (64135f4).
⚠️ Report is 44 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/qcodes/instrument_drivers/QDev/QDac_channels.py	7.14%	13 Missing ⚠️
...odes/instrument_drivers/Keithley/_Keithley_2600.py	0.00%	2 Missing ⚠️
...ent_drivers/QuantumDesign/DynaCoolPPMS/DynaCool.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8354      +/-   ##
==========================================
- Coverage   71.12%   71.09%   -0.04%     
==========================================
  Files         305      305              
  Lines       32010    31944      -66     
==========================================
- Hits        22768    22711      -57     
+ Misses       9242     9233       -9     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[astafan8]

Updated based on review plus two follow-ups:

• Normalization inverted: normalize_snapshot_update now returns the canonical SnapshotUpdate literal (All/Only_invalid/Never); all internal code uses those values, not bool/None. Helper + type are exported from qcodes.metadatable.
• Default is now Only_invalid for snapshot/snapshot_base/print_readable_snapshot across the Metadatable hierarchy. .snapshot() with no argument now refreshes parameters with an invalid cache (documented in 8354.breaking). Measurement's registered-parameter snapshots are pinned to Never to preserve prior behavior.
• Legacy bool/None deprecated via typing_extensions.deprecated overloads on Metadatable.snapshot — static only, no runtime warning. Internal code and docs use only the string values; the mapping is confirmed by tests.
• Docs (Working with snapshots notebook + driver examples) and docstrings updated to the enumeration.

Validation: pyright (src+tests) and mypy -p qcodes clean, pre-commit passes, full suite 3135 passed / 268 skipped.

[Copilot]

🟡 Not ready to approve

Some driver and helper snapshot implementations currently don’t fully honor the new "Only_invalid"/"Never" semantics (leading to unnecessary instrument queries and ineffective update="Never" in at least one case).

Once you've addressed the issues Copilot identified, you can request another Copilot review.

This review doesn't count toward merge requirements. Sign up for the private preview to control whether Copilot approvals count.

Pull request overview

This PR formalizes the snapshot(..., update=...) behavior across the Metadatable hierarchy by introducing a canonical string enum ("All", "Only_invalid", "Never") and switching the default snapshot behavior to refresh only invalid caches.

Changes:

• Introduces SnapshotUpdate + normalize_snapshot_update and updates snapshot/snapshot_base/print_readable_snapshot signatures and docstrings to use the new modes (with deprecated bool/None aliases).
• Updates measurement snapshotting semantics: station snapshots use "Only_invalid", while registered-parameter snapshots are pinned to "Never".
• Refreshes tests, driver examples, notebooks, and newsfragments to use the new string values and documents the behavior change.

File summaries

File	Description
tests/test_snapshot.py	Updates snapshot tests to use update="Never"/"All" instead of bools.
tests/test_instrument.py	Updates instrument snapshot test to use update="All".
tests/parameter/test_snapshot.py	Expands/updates parameter snapshot behavior tests for the new update modes and normalization helper.
tests/parameter/test_parameter_cache.py	Updates cache invalidation test to use update="Only_invalid" (and updates accompanying comment).
tests/parameter/test_parameter_basics.py	Updates parameter basics snapshot tests to use update="All".
tests/parameter/test_multi_parameter.py	Updates multi-parameter snapshot tests to use update="All".
tests/parameter/test_delegate_parameter.py	Updates delegate parameter snapshot test to use update="All".
tests/parameter/test_array_parameter.py	Updates array parameter snapshot tests to use update="All".
tests/parameter/conftest.py	Updates update fixture to parametrize over the new string modes.
tests/drivers/test_ami430_visa.py	Updates driver snapshot tests to use update="All".
tests/drivers/keysight_b1500/b1500_driver_tests/test_b1500.py	Updates driver snapshot test to use update="All".
tests/dataset/test_snapshot.py	Adds a regression test ensuring Measurement station snapshot refreshes only invalid caches.
src/qcodes/station.py	Changes station snapshot default to "Only_invalid" and normalizes update mode.
src/qcodes/parameters/sweep_values.py	Normalizes update mode before snapshotting nested parameter.
src/qcodes/parameters/parameter_base.py	Implements "All"/"Only_invalid"/"Never" semantics for parameter snapshotting via cache validity.
src/qcodes/parameters/delegate_parameter.py	Normalizes update mode in delegate parameter snapshotting.
src/qcodes/parameters/combined_parameter.py	Updates signature/docs for new update modes.
src/qcodes/metadatable/metadatable_base.py	Defines SnapshotUpdate, adds normalize_snapshot_update, and updates Metadatable.snapshot overloads/default.
src/qcodes/metadatable/init.py	Exports SnapshotUpdate and normalize_snapshot_update.
src/qcodes/instrument/visa.py	Updates snapshot update typing/docs to new modes.
src/qcodes/instrument/ip.py	Updates snapshot update typing/docs to new modes.
src/qcodes/instrument/instrument_base.py	Normalizes update mode and updates snapshot/print_readable_snapshot defaults and docs.
src/qcodes/instrument/channel.py	Normalizes update mode and updates snapshot/print_readable_snapshot defaults and docs.
src/qcodes/instrument_drivers/stanford_research/SR86x.py	Updates snapshot update typing/default to "Only_invalid".
src/qcodes/instrument_drivers/QuantumDesign/DynaCoolPPMS/DynaCool.py	Updates a forced refresh call to update="All".
src/qcodes/instrument_drivers/QDev/QDac_channels.py	Updates snapshot defaults/typing and adapts driver-specific snapshot cache logic to string modes.
src/qcodes/instrument_drivers/mock_instruments/init.py	Updates mock driver snapshot default typing.
src/qcodes/instrument_drivers/Keysight/private/Keysight_344xxA_submodules.py	Updates forced refresh call to update="All".
src/qcodes/instrument_drivers/Keysight/keysightb1500/KeysightB1517A.py	Updates snapshot update typing/default to "Only_invalid".
src/qcodes/instrument_drivers/Keithley/_Keithley_2600.py	Updates snapshot update typing/default and forced refresh calls to update="All".
src/qcodes/instrument_drivers/american_magnetics/AMI430_visa.py	Normalizes update mode and switches update is True checks to update == "All".
src/qcodes/instrument_drivers/AimTTi/_AimTTi_PL_P.py	Updates forced refresh call to update="All".
src/qcodes/dataset/measurements.py	Uses station.snapshot(update="Only_invalid") and pins registered-parameter snapshots to update="Never".
docs/examples/driver_examples/Qcodes example with Tektronix AWG70002A.ipynb	Updates example snapshot call to update="All".
docs/examples/driver_examples/Qcodes example with Rohde Schwarz ZNB.ipynb	Updates example snapshot call to update="All".
docs/examples/driver_examples/Qcodes example with Rohde Schwarz SGS100A.ipynb	Updates example snapshot call to update="All".
docs/examples/driver_examples/Qcodes example with Rohde Schwarz RTO 1000 series Oscilloscope.ipynb	Updates example snapshot call to update="All".
docs/examples/driver_examples/Qcodes example with Oxford Mercury iPS.ipynb	Updates example snapshot call to update="All".
docs/examples/driver_examples/Qcodes example with Keysight 344xxA.ipynb	Updates example snapshot call to update="All".
docs/examples/driver_examples/Qcodes example with HP8753D.ipynb	Updates example snapshot call to update="All".
docs/examples/driver_examples/Qcodes example with DynaCool PPMS.ipynb	Updates example snapshot call to update="All".
docs/examples/driver_examples/QCoDeS example with CopperMountain_M5065.ipynb	Updates example snapshot call to update="All".
docs/examples/DataSet/Working with snapshots.ipynb	Documents the new update modes and measurement snapshot behavior.
docs/changes/newsfragments/8354.new	Announces the new update mode enum and normalization helper.
docs/changes/newsfragments/8354.breaking	Documents the default change to "Only_invalid" and how to restore old behavior.

Review details

Comments suppressed due to low confidence (1)

src/qcodes/instrument_drivers/QDev/QDac_channels.py:350

• Instrument-level snapshot_base always calls _update_cache for update="Only_invalid" (update != "Never"), which can make the new default snapshot mode slow even when all channel caches are already valid. Consider only calling _update_cache for "All", or for "Only_invalid" when any cached channel values (e.g. v/vrange/irange) are invalid.

        update = normalize_snapshot_update(update)
        update_currents = self._update_currents and update == "All"
        if update != "Never":
            self._update_cache(readcurrents=update_currents)
            self._get_status_performed = True

• Files reviewed: 45/45 changed files
• Comments generated: 3
• Review effort level: Low

We're testing this review assessment. Please use 👍 or 👎 to tell us if it's correct.

[jenshnielsen]

Please have a look at the open copilot questions. Otherwise looks good.