Drain HTTP/3 response after trailers

[antonfirsov]

Fixes #60118.

Merges the sync and async implementation of Http3RequestStream disposal, since QuicStream.Dispose() does sync over async anyways.

The test changes are built on top of #116113 which is a rather independent and safe change, it would be desireable to merge that PR first.

Note: I wasn't able to reproduce #60118 with the POST Duplex Slow stress case. The server-side PEER_RECEIVE_ABORTED events seen there were results of cancellation and by setting cancelRate=0 they stopped to occur. GetAsync_TrailersWithoutServerStreamClosure_Success reproduces the issue without the product code change.

[Copilot]

Pull Request Overview

This PR unifies sync/async disposal in Http3RequestStream, ensures HTTP/3 responses are drained after trailers, and refactors test helper stream usage.

• Merge sync and async Dispose implementations into a single async-driven flow.
• Introduce DrainResponseAsync to consume leftover response data after trailers.
• Update Http3LoopbackStream in tests to use a Stream property instead of a private field.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Http3RequestStream.cs	Unified Dispose logic, added response-drain task and helper.
Http3LoopbackStream.cs	Replaced _stream field with Stream property and updated refs.

Comments suppressed due to low confidence (2)

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs:581

• There’s a TODO placeholder here; please add a descriptive comment explaining why it's safe to stop reading after the trailing headers and what extensions are skipped.

// TODO: Add comments here.

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs:1423

• Add a clear comment describing why initiating response drain on trailing headers is correct and what kinds of frames may be skipped safely.

// TODO: add proper comment.