test(fc): store from anchor rejects mismatched state root

[mananuf]

🗒️ Description

Adds a fork choice filler asserting that Store.from_anchor aborts when the anchor block's state_root disagrees with hash_tree_root(anchor_state). This is a safety precondition every client must enforce: an anchor pair where the block and state disagree on the post state would corrupt every subsequent block-root-to-state lookup.

Test

tests/consensus/devnet/fc/test_checkpoint_sync.py::test_store_from_anchor_rejects_mismatched_state_root

Builds a valid mid-chain anchor via build_anchor, overrides the anchor block's state_root with an unrelated value, and feeds the pair to the filler. The fixture verifies Store.from_anchor raises AssertionError whose message contains the exact precondition text ("Anchor block state root must match anchor state hash"), pinning the failure to the state-root check rather than any later crash.

Framework extension

Store.from_anchor runs before any step, so the existing per-step valid=False / expected_error mechanism cannot express an expected failure at initialization. Two optional fields are added to ForkChoiceTest to fill this gap:

• anchor_valid: bool = True
• expected_anchor_error: str | None = None

When anchor_valid=False, the fixture:

1. Asserts steps == [] (no step can run if init aborts).
2. Skips the validator pubkey sync that would otherwise silently overwrite the anchor block's state_root to match the state.
3. Wraps Store.from_anchor in try/except and verifies the raised exception's message contains expected_anchor_error.

Defaults preserve existing behaviour: all 545 consensus fixtures regenerate byte-identical.

Serialised as anchorValid / expectedAnchorError via CamelModel. Clients consuming fixtures branch on anchorValid=false and verify their equivalent Store.from_anchor rejects the input.

🔗 Related Issues or PRs

Closes #570

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox -e all-checks

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.