ngtcp2: stabilize recv

[icing]

When receiving on a stream that already failed or has already been closed, return the matching error code without touching the connection. In case the connection shows errors, e.g. the server closed, those errors should not have impact on an already failed/closed stream.

This might mitigate flakiness in pytest 07_13 where unexpected errors occur after a successful upload.

[bagder]

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[augmentcode]

🤖 Augment PR Summary

Summary: Stabilizes HTTP/3 recv handling for the ngtcp2 backend by preferring already-known per-stream outcomes over later connection-level errors.

Changes:

• Updates cf_ngtcp2_recv() to first check stream->xfer_result / stream->closed and return the matching error/status immediately.
• Processes ingress once via cf_progress_ingress() only when the stream is still active, then re-checks xfer_result/closed afterward.
• Leaves the normal “no data yet” behavior unchanged by returning CURLE_AGAIN when the stream remains open.

Technical Notes: Intended to mitigate flakiness (e.g. pytest 07_13) where late connection errors after a successful upload could incorrectly affect an already failed/closed stream.

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