fix(spec-specs): tolerate a missing fork cache at json_loader teardown

[mkzung]

Description

tests/json_loader/conftest.py closes a fork cache in pytest_sessionfinish that its own pytest_sessionstart may never have created. pytest registers a directory's conftest before session start only when that directory is named on the command line; reached by collection it arrives after, so the start hook here never runs while the finish hook still does.

Any run over tests/ that does not name tests/json_loader hits it. The test passes, the session then dies in teardown, prints no summary and exits 1:

$ uv run pytest tests -k test_decode_max_withdrawal_amount
.                                                          [100%]Traceback (most recent call last):
...
KeyError: <_pytest.stash.StashKey object at 0x10de06440>
$ echo $?
1

Guarding the read matches the stash.get(key, None) reads in plugins/forks/forks.py and plugins/consume/simulators/helpers/test_tracker.py. The same command then reports 1 passed, 18066 deselected and exits 0.

test_conftest.py covers both directions: reverting the guard fails the empty-stash test with the same KeyError, and deleting the teardown body outright fails the other on assert cache.closed.

Related Issues or PRs

N/A.

Checklist

• Ran fast static checks to avoid CI fails, see Code Standards & Verifying Changes: just static
• PR title has the form <type>(<area>): <title>, where <type> and <area> come from an appropriate C-<type>, respectively A-<area>, label. The title should match the target squash commit message.

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