fix(http2): make StreamException a StreamTransportException and forward ALPN errors to onError

[mosuem]

Note

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

Summary

Two small error-handling fixes:

1. StreamException (lib/src/sync_errors.dart): used by StreamHandler.processGoawayFrame when aborting streams with id > lastStreamId ("this stream was not processed and can therefore be retried") and by _rejectOversizedHeaders. It implemented Exception directly instead of extending StreamTransportException, so callers that catch TransportException / StreamTransportException as documented by package:http2/transport.dart missed exactly the error that tells them a retry is safe.
2. MultiProtocolHttpServer.startServing (lib/multiprotocol_server.dart): when a client negotiated an unexpected ALPN protocol, startServing threw synchronously inside the _serverSocket.listen data callback instead of forwarding the error to onError when one was provided.

Changes

• lib/src/sync_errors.dart: StreamException extends StreamTransportException, toString() format unchanged. StreamHandler.processStreamFrame catches the concrete subtypes (StreamClosedException, StreamRefusedException, StreamException), so its RST_STREAM error-code mapping is unaffected.
• lib/multiprotocol_server.dart: forward the unexpected-protocol error to onError if provided.

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

goaway-terminates-nonprocessed-streams in test/client_test.dart now also asserts e is StreamTransportException.

Before fix:

00:00 +0 -1: test/client_test.dart: client-tests client-errors goaway-terminates-nonprocessed-streams [E]
  Expected: <Instance of 'StreamTransportException'>
    Actual: StreamException:<StreamException(stream id: 1): Remote end was telling us to stop. This stream was not processed and can therefore be retried (on a new connection).>
     Which: is not an instance of 'StreamTransportException'

After fix:

00:00 +1: All tests passed!

[github-actions]

PR Health

Coverage ⚠️

File	Coverage
pkgs/http2/lib/multiprotocol_server.dart	💔 85 % ⬇️ 5 %
pkgs/http2/lib/src/sync_errors.dart	💚 74 % ⬆️ 2 %

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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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http2	✔️ All dependencies utilized correctly.

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