Fix PipeReader contract violation in DefaultHttpRequestParser

[tobias-tengler]

Closes #9759

[Copilot]

Pull request overview

This PR fixes a PipeReader contract violation in DefaultHttpRequestParser that could leave a final successful ReadAsync un-advanced when parsing fails, triggering intermittent InvalidOperationException (“Reading is already in progress”) during Kestrel request-body draining on HTTP/1.1 (Issue #9759).

Changes:

• Refactors request body reading into a shared ReadFullBodyAsync loop and ensures the final ReadResult is always followed by AdvanceTo(...) via try/finally (even on syntax/parse errors).
• Preserves existing request size enforcement while ensuring AdvanceTo is called on the oversize path.
• Adds tests covering syntax error and canceled read scenarios to validate the reader is left in a readable state after failures.

Reviewed changes

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

File	Description
src/HotChocolate/AspNetCore/src/AspNetCore.Pipeline/Parsers/DefaultHttpRequestParser.cs	Ensures the final successful ReadAsync is always advanced (via finally) and centralizes “read full body” logic to honor PipeReader contract on all exit paths.
src/HotChocolate/AspNetCore/test/AspNetCore.Tests/Serialization/DefaultHttpRequestParserTests.cs	Adds regression tests that attempt subsequent reads after failure/cancel to catch the prior “reading already in progress” behavior.

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