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 94.98%. Comparing base (64cbead) to head (e63c81d).

Additional details and impacted files

@@              Coverage Diff              @@
##           forks/bogota    #3507   +/-   ##
=============================================
  Coverage         94.98%   94.98%           
=============================================
  Files               673      673           
  Lines             40831    40831           
  Branches           3747     3747           
=============================================
  Hits              38785    38785           
  Misses             1429     1429           
  Partials            617      617           

Flag	Coverage Δ
unittests	94.98% <ø> (ø)

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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.

[fselmo]

forks/amsterdam is now frozen, so I moved this PR to forks/bogota and rebased it onto the new base. The rebase applied without conflicts. If you have local commits, please rebase them onto this branch before pushing again.