fix: missing check for unexpected response stream close in append session

[quettabit]

closes #65

[greptile-apps]

Greptile Summary

This PR adds a guard at the end of _run_attempt in the append session to detect when the server closes the response stream while there are still unacknowledged in-flight batches, raising an S2ClientError in that case.

• Adds a post-loop check (if inflight_inputs:) after the response stream is exhausted, surfacing an explicit error instead of silently returning with unacknowledged batches.
• Uses S2ClientError (not a TransportError) because this condition is treated as a protocol violation — the prior review thread confirmed that intentionally bypassing retry machinery is the correct behavior here.

Confidence Score: 5/5

Safe to merge — the fix correctly surfaces a previously silent failure mode with no impact on the normal append path.

The added check fires only after the response loop exits via StopAsyncIteration, at which point no concurrent _body_gen iteration can race against the inflight_inputs read. The chosen exception type is consistent with the other protocol-violation raises already in _run_attempt, and the deliberate decision not to retry this condition was confirmed by the team.

No files require special attention.

Important Files Changed

Filename	Overview
src/s2_sdk/_s2s/_append_session.py	Adds a post-loop inflight check in _run_attempt; logic is correct and consistent with existing protocol-violation error handling in the same function.

Sequence Diagram

sequenceDiagram
    participant C as Client (_body_gen)
    participant S as Server
    participant RL as Response Loop

    C->>S: Send batch (inflight_inputs grows)
    S-->>RL: AppendAck received
    RL->>RL: popleft() from inflight_inputs
    RL->>C: yield AppendAck

    Note over S,RL: Unexpected stream close scenario
    C->>S: Send batch N (inflight_inputs grows)
    S-->>RL: Stream closed (StopAsyncIteration)
    RL->>RL: break out of while loop
    RL->>RL: check: if inflight_inputs (non-empty)
    RL-->>C: raise S2ClientError (protocol violation, no retry)

Loading

_{Reviews (2): Last reviewed commit: "initial commit" | Re-trigger Greptile}

[quettabit]

@greptileai address your comments. pls review again.