[banglcb]

Closes #2606.

ConnectionManager gains an optional clientSettings, passed to ClientTransportConnection.viaSocket for every connection it creates. null (the default) keeps package:http2's defaults, so existing behavior is unchanged. ClientSettings is re-exported from dio_http2_adapter so callers need no direct http2 dependency.

Why one ClientSettings rather than streamWindowSize/concurrentStreamLimit ints as sketched in #2606: the stream window alone does not lift the cap. In package:http2 up to 3.1.0 the connection receive window is fixed at 65535 bytes, so at most 64 KiB is in flight per round trip whatever streamWindowSize is. dart-lang/http#1999 (merged, in http2 3.2.0-wip, not yet on pub.dev) adds Settings.connectionWindowSize and Settings.maxFrameSize. Taking the whole ClientSettings lets users of http2 3.2 set those without another dio API change, and works today with ^2.1.0 (streamWindowSize, concurrentStreamLimit, allowServerPushes). No dependency constraint changes.

For reference, with http2 3.2's settings (streamWindowSize 6 MiB, connectionWindowSize 16 MiB, maxFrameSize 1 MiB), a 4 MiB body from a Go net/http server through 100 ms of added RTT took ~340 ms against ~350 ms for HTTP/1.1, and 7.1 s with the defaults (numbers from dart-lang/http#1997).

Verification: two tests in test/http2_test.dart read the client's first SETTINGS frame off the wire through a local h2c server: with clientSettings it carries SETTINGS_INITIAL_WINDOW_SIZE = 1 MiB (fails when the settings are not passed to viaSocket), without it the setting is absent as before. dart analyze --fatal-infos and dart format are clean for the package; dart test in plugins/http2_adapter passes 104 tests.

Unverified: three existing ALPN tests that drive openssl s_server fail on my Windows machine, identically on unmodified main (fails with HandshakeException when only h2 is advertised..., routes to fallbackAdapter when h2 and http/1.1 are both advertised..., ...through a proxy); I rely on CI for those. I did not run melos run test across the other packages, which this change does not touch.

AI disclosure: implementation, tests and this description were drafted with Claude (Claude Code) and reviewed by me.

New Pull Request Checklist

• I have read the Documentation (not applicable — only the http2_adapter package docs and source were relevant)
• I have read the Agent Contribution Guidelines (required if any part of the change was produced with AI assistance)
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures (three pre-existing openssl-based ALPN tests fail locally on unmodified main too; see Unverified)
• I have updated the CHANGELOG.md in the corresponding package