[http2] Gracefully receive headers on canceled streams

[wchargin]

If a client opens a stream and then cancels it, the server may have already sent response frames back. The HTTP/2 spec says that the client MUST be prepared to receive any such frames. We already handle this correctly for data frames, but prior to this patch, header frames on a canceled stream would cause the whole connection to be torn down. This patch fixes that.

Fixes #1799.

Test Plan:
Unit tests included. Before this change, the new test would fail with "Connection is being forcefully terminated. (errorCode: 1)".

wchargin-branch: http2-headers-after-cancel

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• I’ve reviewed the contributor guide and applied the relevant portions to this PR:
  • Changelog updated
  • Pubspec version revved
  • Tests included

[wchargin]

Hello! This is my first time writing Dart code. I tried to follow local style, and ran dart format, dart fix, and dart analyze. Please let me know if you'd like any modifications; happy to conform to your preferences.

[wchargin]

Hi @brianquinlan, might you be able to please take a look at this PR? I saw that you have a goal of a weekly PR review cadence and I look forward to any feedback you might have. We're currently working around this bug in our servers, and it would be great to get a fix upstreamed.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
http2	Non-Breaking	2.3.1	3.0.1-wip	2.4.0	✔️

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ✔️

File	Coverage
pkgs/http2/lib/src/streams/stream_handler.dart	💚 91 % ⬆️ 1 %

This check for test coverage is informational (issues shown here will not fail the PR).

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources

License Headers ✔️

// Copyright (c) 2025, 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.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/http/example/main.dart

[brianquinlan]

@mosuem Could you take a look at this?

[mosuem]

Will do, thanks for the ping!