fix: unretrieved exceptions reported during Future/Task finalization

[quettabit]

No description provided.

[quettabit]

@greptileai review pls

[greptile-apps]

Greptile Summary

This PR prevents unretrieved Future and Task exceptions from being reported during finalization and consistently consumes exceptions during asynchronous cleanup.

• Adds shared helpers for setting and retrieving future exceptions.
• Retrieves background-task failures through callbacks or gather(..., return_exceptions=True).
• Logs unexpected connection-pool reaper failures.
• Enables asyncio debug mode in the producer and consumer examples.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/s2_sdk/_exceptions.py	Adds centralized helpers that mark future and task exceptions as retrieved.
src/s2_sdk/_producer.py	Applies exception retrieval to producer tickets and background linger tasks while preserving safe linger-task ownership.
src/s2_sdk/_client.py	Retrieves stream and reaper exceptions, logs unexpected reaper failures, and awaits cancelled cleanup tasks.
src/s2_sdk/_append_session.py	Uses the shared helper when failing outstanding append acknowledgements.
src/s2_sdk/_read_session.py	Marks exceptions on caught-up futures as retrieved during closed-session handling.
src/s2_sdk/_batching.py	Consumes completion and cancellation exceptions from the pending record task.
src/s2_sdk/_s2s/_append_session.py	Cancels and gathers acknowledgement-reader helper tasks during cleanup.

_{Reviews (3): Last reviewed commit: "initial commit" | Re-trigger Greptile}

[quettabit]

@greptileai addressed your comments. pls review again.