test: cover checkpoint merge gamma mismatch

[AztecBot]

Summary

Checkpoint merge already compares the full final blob challenge struct between sibling checkpoint rollups, but the local negative tests only mutated z. This adds the missing gamma mismatch regression so the checkpoint-merge suite catches any future narrowing of that conservation check.

Fix

• Add a mismatch_final_blob_challenges_gamma negative test next to the existing z mismatch test.
• Mutate the right checkpoint rollup's final_blob_challenges.gamma and assert the existing checkpoint-merge failure message.

Testing

• PATH=/home/aztec-dev/aztec-packages/noir/noir-repo/target/debug:$PATH nargo test --package rollup_lib checkpoint_merge::tests::consecutive_checkpoint_rollups_tests::mismatch_final_blob_challenges_gamma --exact --silence-warnings

Issues

• Closes AztecProtocol/aztec-claude#1615 — checkpoint merge lacks a local gamma mismatch regression for final blob challenges

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Created by claudebox · group: rollup-audit-remediation