fix(http2): keep flow-controlled end-stream DATA alive until it is flushed (#1998)

[mosuem]

Note

This PR was generated by an AI coding agent (Jetski) on behalf of @mosuem.

Summary

Fixes #1998: sendData(bytes, endStream: true) with more bytes than the peer's flow-control window stalled forever, because the stream was torn down before the flow-controlled remainder could be sent.

Two things went wrong:

1. In StreamMessageQueueOut.enqueueMessage, if (message.endStream) startClosing() ran before _messages.addLast(message). startClosing() synchronously calls onCheckForClose(), which saw an empty _messages and closed the queue while the DataMessage was still waiting for WINDOW_UPDATE credit.
2. In StreamHandler._handleEndOfStreamRemote, when the peer sent END_STREAM on a stream that was already HalfClosedLocal (the local side had called sendData(..., endStream: true)), the stream was removed from _openStreams immediately, even if stream.outgoingQueue still had DATA waiting on credit. Subsequent stream-level WINDOW_UPDATE frames from the peer were then ignored (RFC 9113 Section 6.9: a WINDOW_UPDATE on a closed stream is discarded), so the remaining data could never be sent.

Changes

• lib/src/flowcontrol/stream_queues.dart: Call startClosing() after _messages.addLast(message), so onCheckForClose() only closes the queue once _messages has drained.
• lib/src/streams/stream_handler.dart: In _handleEndOfStreamRemote, only remove the stream from _openStreams if stream.outgoingQueue.wasClosed; otherwise _cleanupClosedStream removes it once the outgoing queue has drained.

Test Verification (Fails Before $\rightarrow$ Passes After)

Added send-data-exceeding-window-with-end-stream-completes in test/transport_test.dart (server and client each send 100 000 bytes with endStream: true through a 65 535-byte window; the second case has the server respond with END_STREAM before the upload has drained).

Before fix:

00:05 +0 -1: transport-test flow-control send-data-exceeding-window-with-end-stream-completes [E]
  TimeoutException after 0:00:05.000000: Test timed out after 5 seconds.

After fix:

00:00 +1: All tests passed!

[github-actions]

PR Health

Coverage ✔️

File	Coverage
pkgs/http2/lib/src/flowcontrol/stream_queues.dart	💚 90 %
pkgs/http2/lib/src/streams/stream_handler.dart	💚 91 % ⬆️ 0 %

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// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
http2	Non-Breaking	3.1.0	3.2.0-wip	3.2.0-wip	✔️

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Package	Status
http2	✔️ All dependencies utilized correctly.

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