Implement SocketsHttpHandler.InitialHttp2MaxConcurrentStreams

[MihaZupan]

Implements #132457 for .NET 12. Potential backports to 11 will follow later.

0 is technically also a valid value for this property, but I blocked it for now since it requires a bit of extra code (see bae3040), while I don't see a good realistic scenario rn. We can always add support for it back later if someone cares.

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[Copilot]

Pull request overview

Adds a new SocketsHttpHandler configuration knob to cap how many HTTP/2 streams a new connection may open before the peer’s initial SETTINGS frame is observed, and adjusts the existing “last seen” SETTINGS_MAX_CONCURRENT_STREAMS behavior to respect that cap. This fits into the System.Net.Http connection-pooling and HTTP/2 stream management code paths.

Changes:

• Introduces SocketsHttpHandler.InitialHttp2MaxConcurrentStreams (default 100, must be >= 1) and threads it through connection settings/pool initialization.
• Updates HTTP/2 SETTINGS processing to only memorize server-advertised max-concurrent-streams values when they are lower than the configured initial cap.
• Adds functional tests validating round-tripping, validation, and the before-SETTINGS behavior/memoization behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs	Adds API validation/roundtrip tests and HTTP/2 loopback tests covering pre-SETTINGS stream limiting and memoization rules.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/SocketsHttpHandler.cs	Adds the new public property with validation and XML docs.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionSettings.cs	Stores/clones the new initial max concurrent streams setting.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Connection.cs	Removes the hardcoded initial stream cap and updates SETTINGS_MAX_CONCURRENT_STREAMS handling/memoization logic.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.cs	Initializes per-pool “last seen” HTTP/2 max concurrent streams from the new setting.
src/libraries/System.Net.Http/src/System/Net/Http/HttpHandlerDefaults.cs	Introduces the default constant (100) for the new property, with rationale.
src/libraries/System.Net.Http/src/System/Net/Http/BrowserHttpHandler/SocketsHttpHandler.cs	Adds the corresponding browser stub property throwing PNSE.
src/libraries/System.Net.Http/ref/System.Net.Http.cs	Exposes the new public API surface in the ref assembly.

[wfurt]

LGTM

[MihaZupan]

/backport to release/11.0

[github-actions]

Started backporting to release/11.0 (link to workflow run)