fix: queue sentinels in finally can hang append session during task cancellation

[quettabit]

closes #118

[greptile-apps]

Greptile Summary

This PR prevents append-session cancellation from entering potentially blocking queue operations in finally blocks.

• Sends input and acknowledgement sentinels only after normal producer completion.
• Preserves sentinel delivery for both normal retry-loop exit paths.
• Allows TaskGroup cancellation and failures to propagate without waiting for queue capacity.

Confidence Score: 5/5

The PR appears safe to merge; normal completion remains correctly signaled and abnormal termination now relies on TaskGroup cancellation rather than potentially blocking cleanup writes.

Both normal retry-loop exits enqueue the acknowledgement sentinel exactly once, while producer or retry failures cancel outstanding queue waits and propagate through the public append-session error handling.

Important Files Changed

Filename	Overview
src/s2_sdk/_s2s/_append_session.py	Moves queue sentinel writes out of cancellation-sensitive finally blocks while preserving normal append-session termination.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    I[Input producer] -->|append inputs| IQ[Input queue]
    IQ --> R[Retrying append task]
    R -->|append acknowledgements| AQ[Acknowledgement queue]
    AQ --> C[Generator consumer]
    I -->|normal completion: None| IQ
    R -->|normal completion: None| AQ
    E[Failure or cancellation] --> TG[TaskGroup cancellation]
    TG --> I
    TG --> R
    TG --> C

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_{Reviews (1): Last reviewed commit: "initial commit" | Re-trigger Greptile}