Support for the WT_DRAIN_SESSION frame to initiate draining

[jesup]

Support for the WT_DRAIN_SESSION frame to initiate draining for a specific session

[Copilot]

Pull request overview

Adds WebTransport support for the WT_DRAIN_SESSION capsule/frame so endpoints can initiate graceful draining of a specific WebTransport session and surface this as an application event.

Changes:

• Introduces a DrainSession WebTransport frame (WT_DRAIN_SESSION) with encode/decode + frame reader tests.
• Wires a new “draining” signal through the extended-connect session protocol and event pipelines to client/server application events.
• Adds test-only helpers and new integration tests covering client↔server draining behavior.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
neqo-http3/src/server_events.rs	Adds public WebTransportServerEvent::Draining and event insertion helper.
neqo-http3/src/server_connection_events.rs	Adds internal per-connection WebTransportEvent::Draining and emits it from ExtendedConnectEvents.
neqo-http3/src/server.rs	Maps connection draining events to public server events; adds test helper to send drain capsule.
neqo-http3/src/frames/wtframe.rs	Implements WT_DRAIN_SESSION frame type, encode/decode logic, and unit tests.
neqo-http3/src/frames/tests/reader.rs	Refactors CloseSession frame assertions to safer pattern matching.
neqo-http3/src/features/extended_connect/webtransport_session.rs	Handles received DrainSession frame by marking session draining and emitting draining events.
neqo-http3/src/features/extended_connect/tests/webtransport/sessions.rs	Adds end-to-end tests for client/server drain behavior; updates draft links.
neqo-http3/src/features/extended_connect/mod.rs	Extends ExtendedConnectEvents trait with session_draining.
neqo-http3/src/connection_server.rs	Adds test-only server handler hook to send a drain capsule.
neqo-http3/src/connection_client.rs	Adds test-only client helper to send a drain capsule.
neqo-http3/src/connection.rs	Adds test-only connection helper to write WT_DRAIN_SESSION on a session send stream.
neqo-http3/src/client_events.rs	Emits Http3ClientEvent::WebTransport(WebTransportEvent::Draining) for session draining.

[larseggert]

@jesup all of these PRs need to be rebased so CI runs.

[jesup]

This PR is part of a stack of 13 bookmarks:

1. trunk()
2. users/jesup/exportkey
3. users/jesup/draining
4. users/jesup/sub_protocol
5. users/jesup/sendGroup
6. users/jesup/stats
7. users/jesup/highwatermarks
8. users/jesup/sendgroup_sendorder_tracking
9. users/jesup/stats_overhead
10. users/jesup/avoid_PMTUD
11. users/jesup/anticipated_streams
12. users/jesup/waitUntilAvailable
13. users/jesup/atomicWrite
14. users/jesup/WT_DRAIN_SESSION ← this PR

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Created with jj-stack

[github-actions]

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