Fix pending request cleanup and improve IO fairness

[anaslimem]

Summary
This PR fixes a pending-response leak and improves IO fairness in the RPC layer. It introduces a PendingRequest future that cleans up on drop, switches from biased to unbiased select in the IO loop, and adds a targeted edge-case test to verify cleanup behavior. A small test-only accessor was added to support the new test.

Changes

• Added PendingRequest future with Drop cleanup to prevent pending_responses from growing when requests are canceled or dropped.
• Replaced select_biased! with futures::select! in the IO loop to avoid incoming starvation under heavy outgoing traffic.
• Downgraded “unknown response id” log to debug since dropped requests are now expected.
• Added test_pending_response_cleanup_on_drop to validate cleanup.
• Added a test-only pending_response_count() accessor.

Why

• Long‑lived connections could leak memory if a caller dropped request futures or a peer never responded.
• Biased selection could starve incoming reads under sustained outgoing load, delaying responses and exacerbating pending buildup.