Context

apply_consolidation_merge in crates/zeph-memory/src/consolidation.rs uses a SQLite transaction. BLOCKER-01 from the architect critique was addressed (transaction wrapping), but the failure/rollback path has no automated test.

What to test

Inject a failure mid-transaction (e.g., mock pool to return an error after the first INSERT) and assert:

• The memory_consolidation_sources table has no rows for the attempted merge
• The original messages have consolidated = false
• The function returns Err, not Ok

Acceptance criteria

A #[tokio::test] in consolidation.rs that exercises the rollback path and passes.

Discovered in PR #2358 tester review.