Fix Hypercorn's HTTP/3 handling of informational responses

[code-1py]

Summary

Fix Hypercorn's HTTP/3 handling of informational responses such as 103 Early Hints. aioquic currently advances its header send state for every HEADERS frame, including interim responses, as described in aioquic issue #656. This can leave the stream in the wrong state for final response headers and response DATA.

This change adds a Hypercorn-side workaround while the upstream fix in aioquic pull request #657 is pending. With this change, Hypercorn can send HTTP/3 informational responses without prematurely advancing the headers send state.

Changes

• Add H3Wrapper, a subclass of aioquic's H3Connection, with a custom send_headers() implementation.
• Detect Hypercorn InformationalResponse events in H3Protocol.stream_send() and pass is_informational=True to the wrapper.
• Preserve the headers send state for informational headers; advance it for final response headers so DATA and optional trailers can follow.
• Reject informational headers with end_stream=True and informational headers sent after final response headers.

Tests

• Verify one or multiple informational responses leave the send state in INITIAL, then final response headers advance it to AFTER_HEADERS and DATA can end the stream.
• Verify informational headers cannot end the stream.
• Verify informational headers cannot be sent after final response headers.

References

• aioquic issue #656
• aioquic pull request #657

Related issue: #380