test(st): attestation with already-justified target is silently skipped

[menawar]

Summary

• Add test_attestation_with_already_justified_target_is_silently_skipped to test_justification.py
• Justifies slot 1 via supermajority (validators 0–2), then includes a late attestation from validator 3 targeting the same already-justified slot 1
• Verifies the duplicate attestation is silently skipped: no error, no double-counting, no pending vote residue

🔗 Related Issues or PRs

closes #576

Design notes

The test uses the simplest possible chain (4 validators, 3 slots) to isolate the skip path at State.process_attestations where justified_slots.is_slot_justified(...)
returns True and the attestation is dropped via continue.

Post-state assertions confirm:

• latest_justified_slot stays at 1 (no regression)
• justifications_roots and justifications_validators are empty (pending votes were cleared on justification and the skipped attestation created no new ones)
• justified_slots is unchanged (no double-marking)

Test plan

• uv run fill --fork=devnet --clean -n auto -k test_attestation_with_already_justified passes
• Full suite uv run fill --fork=devnet --clean -n auto passes (no regressions)
• uvx tox -e all-checks — ruff, ty, codespell, mdformat all pass