fix(lean): reject malformed justification state with typed errors

[perfogic]

What was wrong?

Address leanSpec #1178

TLDR:

• is_justifiable_after is now total: below-finalized slots return false instead of erroring, and the fixture runner supports signed deltas for those cases.
• process_attestations now rejects malformed justification bookkeeping before unpacking, including vote-list length mismatch and zero-hash roots.

How was it fixed?

• Made is_justifiable_after total by returning false for slots before finalization.
• Updated the justifiability runner to support signed deltas from leanSpec 1178.
• Added malformed justification-state guards for vote-list length mismatch and zero-hash roots.
• Added direct tests for below-finalized slots and attestation senarios.

To-Do

• I have read CONTRIBUTING.md.
• This PR title follows conventional commits.