fix(fill): re-anchor start_block under --no-reset-between-tests

[jochem-brouwer]

Description

Problem

--no-reset-between-tests keeps the chain moving between fills, but the
session anchor never moves with it.

_session_pre_run captures start_block once, from latest, after the
global setup:

@pytest.fixture(scope="session", autouse=True)
def _session_pre_run(...):
    ...
    # 4. Capture start block (head after global setup).
    start_block = eth_rpc.get_block_by_number("latest")
    client_backend.start_block = start_block

That is the only production write to client_backend.start_block. Every
test then chains its first block from it, in make_stateful_fixture.

Normally the invariant holds because _reset_chain_between_tests rewinds
the chain back to that block after each test. Under
--no-reset-between-tests the rewind is skipped and nothing takes its
place, so the anchor goes stale as soon as the first test builds a block.

The nonce side of the flag was already adapted — worker_key reads at
latest under the flag, with a comment explaining the inversion. The
anchor side was missed.

Related Issues or PRs

Stacked on #3304, which introduces the --no-reset-between-tests flag this fixes. Until that PR merges, the diff here includes its commits; the change in this PR is the single commit on top, by @skylenet. See also jochem-brouwer#6

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.

Cute Animal Picture

[jochem-brouwer]

This builds on top of #3304 so that should first be merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.90%. Comparing base (786c0d4) to head (4dd4b6a).
⚠️ Report is 34 commits behind head on forks/amsterdam.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           forks/amsterdam    #3507      +/-   ##
===================================================
+ Coverage            93.54%   93.90%   +0.35%     
===================================================
  Files                  624      624              
  Lines                37074    36905     -169     
  Branches              3394     3326      -68     
===================================================
- Hits                 34681    34655      -26     
+ Misses                1644     1569      -75     
+ Partials               749      681      -68     

Flag	Coverage Δ
unittests	93.90% <ø> (+0.35%)	⬆️

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[danceratopz]

This builds on top of #3304 so that should first be merged.

What do you think about adding that to the top of the PR description and moving this to draft?

[jochem-brouwer]

@danceratopz that PR (#3304) adds the --no-rest-between-tests flag which we slightly alter here.

I have drafted this PR because from https://github.com/jochem-brouwer/execution-specs/pull/6/commits we need to add two more commits 3d072ce and e269bb4. The latter somehow raises a merge conflict if I cherry-pick it. Will look into it.