test: cover Redis retry and DLQ recovery

[eshulman2]

Summary

• add deterministic single-batch retry dispatch for integration testing
• verify transient consumer failures retry successfully and clear Redis pending state
• verify persistent failures exhaust retries, move to the DLQ, and clear pending state
• acknowledge the original stream entry when retry exhaustion transfers ownership to the DLQ
• keep retry unit tests hermetic by injecting a mocked Redis client

Bug fixed

The retry poller moved exhausted messages to the dead-letter queue but did not acknowledge their original Redis Stream entry. This left a permanent PEL record after terminal failure.

Verification

• 22 retry, consumer, and retry-queue unit tests passed
• 11 real-Redis retry/DLQ and queue integration tests passed
• Ruff lint and formatting checks passed for source and changed tests