Fix HTTP/2 Pings not being sent/enforced in multiple scenarios

[MihaZupan]

Fixes #120179

There were 3 scenarios where we'd stop sending pings / checking whether the PONG delay was hit, as called out in the issue

1. The connection reached its stream limit and was temporarily removed from the available list.
2. We received a GOAWAY frame indicating that existing requests will be processed. The first thing we do when receiving the frame is to mark the connection as shut down, and remove it from the available pool.
3. If the SocketsHttpHandler instance is disposed. Today any existing requests on the instance will continue as normal, but we're also stopping the heartbeat timer and therefore breaking pings. This case could be looked at as user error to a degree though.

The issue boils down to using the same list of connections that are still available with capacity, and disposing the timer prematurely.

This fixes that by tracking all H2 connections that still need pings separately, and avoids disposing the timer until that list is also empty instead of immediately on handler disposal.

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[Copilot]

Pull request overview

This PR adjusts SocketsHttpHandler’s HTTP/2 keep-alive ping/timeout enforcement so it continues to function even when a connection is no longer “available” in the pool (e.g., draining after GOAWAY or temporarily removed due to stream limits), and so pings can continue for in-flight requests after handler disposal.

Changes:

• Track all live HTTP/2 connections that still require heartbeats separately from the “available” connection list, and drive ping heartbeats from that tracking list.
• Keep the HTTP/2 heartbeat timer alive past handler disposal and stop it only once the manager is disposed and no heartbeat-relevant connections remain.
• Add functional tests covering GOAWAY, max concurrent streams, and handler disposal scenarios for HTTP/2 keep-alive pings.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.Http2KeepAlivePing.cs	Adds OuterLoop functional coverage for GOAWAY/max-streams/handler-dispose keep-alive ping behavior, plus helper for awaiting pings.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPoolManager.cs	Changes heartbeat timer lifetime so pings can continue for in-flight requests after Dispose; timer self-terminates when safe.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Connection.cs	Registers HTTP/2 connections for pool heartbeat across full lifetime; gates pings until setup completes; unregisters on teardown.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.Http2.cs	Introduces a heartbeat-specific HTTP/2 connection tracking list and updates pool HeartBeat() to use it.
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.cs	Prevents pool cleanup from disposing a pool that still has draining HTTP/2 connections needing heartbeats.