Atomically send GOAWAY during HTTP/2 abort

[adityamandaleeka]

Fix a race in Http2Connection.Abort where ProcessRequestsAsync cleanup could call _frameWriter.Complete() between WriteGoAwayAsync and Abort(), causing GOAWAY to be dropped.

Add Http2FrameWriter.AbortWithGoAway() and use it from Http2Connection.Abort() to perform GOAWAY write/flush and abort under one lock acquisition.

I'm intentionally not adding a regression test for this because the amount of timing nonsense you have to do to simulate it defeats the purpose of the test. I noticed this happening in CI though so hopefully we'll see less flakiness here.

[Copilot]

Pull request overview

This PR fixes a race condition in HTTP/2 connection abort handling where a GOAWAY frame could be lost. The race occurred when ProcessRequestsAsync cleanup called _frameWriter.Complete() between the WriteGoAwayAsync and Abort() calls, causing the _completed flag to be set to true and the GOAWAY frame to be discarded.

Changes:

• Introduced Http2FrameWriter.AbortWithGoAway() method to atomically write GOAWAY frame and abort connection under a single lock acquisition
• Modified Http2Connection.Abort() to use the new atomic method when closing the connection for the first time
• Added detailed documentation explaining the race condition and the fix

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Servers/Kestrel/Core/src/Internal/Http2/Http2FrameWriter.cs	Added new AbortWithGoAway() method that atomically writes GOAWAY frame and performs connection abort under single lock to prevent race condition
src/Servers/Kestrel/Core/src/Internal/Http2/Http2Connection.cs	Updated Abort() method to use new atomic AbortWithGoAway() when TryClose() succeeds, and regular Abort() otherwise

[adityamandaleeka]

ha, this ran into the flakniess fixed in #65456