Add leanSpec test vector for zero hash in justifications_roots

[pablodeymo]

PR #216 added a precondition check in process_attestations that rejects blocks when justifications_roots contains H256::ZERO, matching the spec assertion at state.py L389-391.

A test vector in leanSpec should be added to exercise this path — a fixture with a zero hash in justifications_roots that expects the block to be rejected.

Reference: #216