test(fc): equivocating proposer tests

[d4m014]

🗒️ Description

• Add test_equivocating_proposer_two_blocks_at_same_slot: both equivocating blocks are accepted by the store and head resolves via lexicographic tiebreaker when weights are equal
• Add test_equivocating_proposer_with_split_attestations: validators split attestations across equivocating forks, verifies the 2 vs 2 tiebreaker phase and the weight-driven head flip when a third validator switches to fork B (3 vs 2)

🔗 Related Issues or PRs

Closes #561, closes #562.

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox

• Considered adding appropriate tests for the changes.