…lassification

When snapshot_nonfinalized_state() returns StillSyncingFinalizedState,
best_chaintip passes through `source.get_block(Height(validator_finalized_height))`
and emits `database_hole` on Ok(None) — categorising a transient
validator-side miss as zaino-owned corruption.

Adds a regression test that synthesizes that branch by claiming a
validator_finalized_height above the mockchain tip and asserts the
resulting error is not a database_hole. Fails today; pins the correct
behaviour once the fix lands.

Refs #1047.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>