fix: pending streams not failed when GOAWAY is received

[quettabit]

closes #44

[quettabit]

@greptileai review pls

[greptile-apps]

Greptile Summary

This PR fixes a bug where streams reserved via reserve_stream() but not yet promoted to active streams (i.e., send_headers not yet called) were left in a hanging state when a GOAWAY frame was received from the server with a non-null last_stream_id.

• Root cause fixed: The ConnectionTerminated handler already iterated _streams to fail those with IDs above last_stream_id, but _pending_streams — entries that have no stream ID yet and are therefore guaranteed unprocessed by the server — were never touched in that branch.
• Fix: Two lines mirror the existing _fail_all_streams pattern by iterating _pending_streams.values() and calling _fail_stream on each, which idempotently sets the error and unblocks any awaiting coroutines.

Confidence Score: 5/5

Safe to merge — the fix is small, targeted, and consistent with the existing stream-failure pattern.

The change adds two lines that mirror the already-tested _fail_all_streams pattern. Pending streams have no assigned stream ID, so they are by definition never processed by the server; failing them on GOAWAY is always correct. The iteration is safe under _write_lock, _fail_stream is idempotent, and send_headers already checks state.error before promoting a pending stream, so the error surfaces cleanly to callers.

No files require special attention.

Important Files Changed

Filename	Overview
src/s2_sdk/_client.py	Added two lines to the GOAWAY handler that fail all pending (pre-header-send) streams; correctly mirrors the existing _fail_all_streams pattern and is safe under the _write_lock.

Sequence Diagram

sequenceDiagram
    participant Caller
    participant Connection
    participant H2

    Caller->>Connection: reserve_stream()
    Connection-->>Caller: state (added to _pending_streams)

    note over Connection,H2: GOAWAY received before send_headers()

    H2-->>Connection: "ConnectionTerminated (last_stream_id=N)"
    Connection->>Connection: _handle_event()
    alt last_stream_id is not None
        loop "_streams where stream_id > N"
            Connection->>Connection: _fail_stream(state, err)
        end
        loop _pending_streams (NEW in this PR)
            Connection->>Connection: _fail_stream(state, err)
        end
    else last_stream_id is None
        Connection->>Connection: _fail_all_streams(err)
    end

    Caller->>Connection: send_headers(state, ...)
    Connection-->>Caller: raises state.error (ProtocolError)

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