Allow creating H2C connections over HTTP CONNECT proxy tunnels

[MihaZupan]

Closes#99689

[MihaZupan]

/azp run runtime-libraries-coreclr outerloop

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[Copilot]

Pull request overview

This PR updates SocketsHttpHandler’s proxy connection pooling so that cleartext HTTP/2 (h2c) requests sent via an HTTP(S) proxy can be carried over a CONNECT tunnel (similar to existing non-secure WebSocket proxy tunneling support), enabling HTTP/2 in scenarios that previously fell back to HTTP/1.1-only proxy forwarding.

Changes:

• Route cleartext HTTP/2 proxy requests to the ProxyTunnel connection kind (CONNECT tunneling) when HTTP/2 is required/preferred.
• Enable HTTP/2 pooling/usage for HttpConnectionKind.ProxyTunnel.
• Add a functional test validating when CONNECT tunneling is used vs. absolute-form forwarding through the proxy.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs	Adds coverage for CONNECT tunneling behavior for HTTPS and cleartext HTTP/2 proxy scenarios.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPoolManager.cs	Adjusts connection key selection to use CONNECT tunneling for cleartext HTTP/2 through HTTP(S) proxies.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionKind.cs	Updates enum comments to reflect HTTP/1.1-only proxy forwarding vs. CONNECT tunneling use cases (including h2c).
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.Http2.cs	Allows pooled HTTP/2 connections for ProxyTunnel pools (debug invariant update).
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.cs	Enables HTTP/2 for ProxyTunnel pools by no longer force-disabling it in the constructor switch.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

[wfurt]

LGTM