test: cover audit retention worker lifecycle

[Chris0Jeky]

Refs #1452

Summary

• Exercise AuditRetentionWorker through StartAsync and its real ExecuteAsync loop.
• Prove the first cleanup receives the configured batch size and a bounded retention cutoff.
• Verify the worker heartbeat and bounded StopAsync cancellation while the configured idle delay is pending.

Verification

• AuditRetentionWorkerTests: 5/5
• Mutation check: bypassing the production cleanup call caused the new test to time out; production code was restored and the class returned to 5/5.
• git diff --check: passed

Two coordinator review passes completed for lifecycle correctness and scope/test quality.

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