Http3RequestStream: read EOS after trailers

[antonfirsov]

This fixes the primary bug captured in #60118 without attempting to drain the stream and without touching the Dispose logic.

We expect trailers to be sent in a single HEADER frame, which is read by ReadHeadersAsync but that method doesn't read the EOS. Despite of that, we set _responseDataPayloadRemaining = -1 afterwards

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs

Lines 577 to 583 in 27c8fe0

	goto case null;
	case null:
	// Done receiving: copy over trailing headers.
	CopyTrailersToResponseMessage(_response!);
	_responseDataPayloadRemaining = -1; // Set to -1 to indicate EOS.
	return;

which then makes Http3RequestStream pretend that it reached EOS in subsequent read calls without ever trying to issue the read to the underlying QuicStream:

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs

Lines 1340 to 1344 in 0d628da

	if (_responseDataPayloadRemaining == -1)
	{
	// EOS -- this branch will only be taken if user calls Read again after EOS.
	return false;
	}

This can be easily fixed by issuing one more read after reading the headers.

[Copilot]

Pull Request Overview

This PR fixes a bug where Http3RequestStream stopped reading the underlying QUIC stream after processing trailers, causing subsequent reads to immediately report end-of-stream. It refactors trailing-header handling into a new ProcessTrailersAsync method that issues one more read to detect EOS, and it updates both DrainContentLength0Frames and ReadNextDataFrameAsync to use this helper. A new regression test GetAsync_TrailersWithoutServerStreamClosure_Success is added to cover scenarios where trailers arrive without an explicit server stream closure.

Reviewed Changes

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

File	Description
src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs	Extracted trailer processing into ProcessTrailersAsync and invoked an extra read to consume EOS.
src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs	Added a new theory-based Http3 regression test for reading trailers before stream closure; removed unused usings.

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[antonfirsov]

/azp run runtime-libraries stress-http

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[ManickaP]

LGTM, thanks!