fix(testing): remove ignored StateTest engine_api_error_code

[junbyjun1238]

🗒️ Description

Remove engine_api_error_code from StateTest.

StateTest currently exposes engine_api_error_code, but the value is silently dropped during the StateTest -> Block conversion in state.py. The native state-fixture path does not consume it either, so the current API advertises a field that is effectively ignored.

This field no longer matches the current codebase's abstraction:

• engine_api_error_code is modeled as block/payload metadata in blockchain.py and fixtures/blockchain.py
• all current in-repo usages attach it to Block(...), not StateTest(...)
• unlike block_exception, it is not wired through _generate_blockchain_blocks() and has no in-tree StateTest(...) call sites

Removing the field makes the API honest and avoids a footgun where a test author can specify an Engine API error expectation that is silently ignored. With BaseTest configured as extra="forbid", unsupported usage will fail fast at model construction rather than being silently ignored.

🔗 Related Issues or PRs

N/A.

✅ Checklist

• All: Ran fast static checks to avoid unnecessary CI fails:

  just static

• All: PR title adheres to the repo standard and starts with type(scope):.
• All: Considered updating the online docs in ./docs/.
• All: Set appropriate labels for the changes (only maintainers can apply labels).

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