fix: missing input exhaustion check before append session completion

[quettabit]

#115 (comment)

[greptile-apps]

Greptile Summary

This PR prevents an append attempt from reporting successful completion when its response stream closes before the request input source has been exhausted.

• Adds an explicit completion invariant after acknowledgements and reconnect advice are handled.
• Updates the reconnect test transport to consume the second attempt’s request body through exhaustion.
• Preserves the existing behavior for advised reconnects and responses that close with unacknowledged batches.

Confidence Score: 5/5

The PR appears safe to merge and correctly prevents premature response closure from being mistaken for successful append-session completion.

The new check runs after outstanding acknowledgements and reconnect advice are resolved, while request-input exhaustion is recorded before the transport can finish a normal request body; no actionable failure remains.

Important Files Changed

Filename	Overview
src/s2_sdk/_s2s/_append_session.py	Adds the missing input-exhaustion invariant before an append attempt can complete successfully.
tests/test_append_session.py	Updates the mock streaming transport to exhaust request content on the final reconnect attempt.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Response stream closes] --> B{Unacknowledged batches?}
    B -->|Yes| C[Fail append session]
    B -->|No| D{Reconnect advised?}
    D -->|Yes| E[Reconnect attempt]
    D -->|No| F{Input source exhausted?}
    F -->|No| C
    F -->|Yes| G[Complete session]

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