refactor(test-fill): use the shared sync-payload builder for sync fixtures

[danceratopz]

Description

Note

This PR is stacked on #3364. Until #3364 lands, please review only this PR's final commit. Once #3364 merges, this PR will be retargeted to forks/amsterdam.

The blockchain_test_engine_x and blockchain_test_sync formats now build their framework-generated sync payloads through the same build_sync_payload helper. The sync format previously built its singular syncPayload inline, without the per-test salt or the checks and defined failure behavior introduced for EngineX by #3364.

For the sync format's valid linear chains:

G → T₁ … Tₙ → S*

G is genesis, T₁…Tₙ are test payloads, S is the empty sync payload, and * marks the payload announced to the sync client. The sync payload is appended above the test-chain head and stored separately in the singular syncPayload field. The test's engineNewPayloads, lastblockhash, and post-state assertion remain unchanged.

Behavioral consequences for blockchain_test_sync fixtures:

1. The sync payload's extraData now carries a 16-byte digest of the test ID, giving it a test-specific blockHash. All nine existing fixtures of this format therefore change in extraData, the resulting blockHash, and fixture metadata; their parent, state root, and fee fields remain unchanged.
2. The singular syncPayload remains mandatory. --no-sync-block and a test's sync_block=False affect only the EngineX format's optional syncPayloads list.
3. If the framework cannot represent or evaluate a child above the test-chain head, the sync-format fill fails with an error naming the conflict. An EngineX fixture can omit one optional sync payload; the sync format cannot serve its consumer without its required one.
4. The format continues to support only valid linear chains. The fill now rejects both expected-invalid test payloads and Engine API error-code assertions anywhere in the sequence, including cases where a later payload gives the filler a valid head.

For implementation and format details, read Sync Payloads first, followed by the blockchain_test_sync format reference.

Validation:

• just static passes.
• The testing-package suite passes: 2,097 passed, 29 skipped, 4 xfailed, and 2 xpassed.
• A before/after corpus comparison confirms that all nine existing sync fixtures differ only in the salted extraData, its resulting blockHash, and fixture metadata.
• A consume sync smoke test using geth as both clients passes all 9 fixtures in 33 seconds; each sync client progresses from SYNCING to VALID, and the head state is verified through eth_getBlockByHash.

Related Issues or PRs

Depends on #3364.

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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.53%. Comparing base (707458c) to head (e4bd1bb).

Additional details and impacted files

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##           experiments/wirex-fill    #3405   +/-   ##
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  Files                         624      624           
  Lines                       37074    37074           
  Branches                     3394     3394           
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  Hits                        34679    34679           
  Misses                       1645     1645           
  Partials                      750      750           

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