[QUIC] Throw on write after closed writing side

[ManickaP]

Fixes #121619

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[Copilot]

Pull request overview

This PR adjusts System.Net.Quic.QuicStream.WriteAsync to throw when callers attempt to write after the stream’s writing side has been gracefully completed, and adds a functional regression test to lock in the behavior. It also includes minor doc-comment grammar fixes and adds a new localized error string.

Changes:

• Update QuicStream.WriteAsync to throw InvalidOperationException when a previously returned write ValueTask is already completed successfully (intended to represent a closed writing side).
• Add a new functional test validating WriteAsync throws after CompleteWrites() + WritesClosed.
• Fix small “is/if” typos in internal task source XML docs and add a new resx string for the new exception message.

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.Quic/src/System/Net/Quic/QuicStream.cs	Changes WriteAsync behavior for post-write-closure calls to throw instead of returning a successful completed ValueTask.
src/libraries/System.Net.Quic/tests/FunctionalTests/QuicStreamTests.cs	Adds regression coverage for writing after the writing side has closed.
src/libraries/System.Net.Quic/src/System/Net/Quic/Internal/ValueTaskSource.cs	Fixes XML doc grammar (“is” → “if”).
src/libraries/System.Net.Quic/src/System/Net/Quic/Internal/ResettableValueTaskSource.cs	Fixes XML doc grammar (“is” → “if”).
src/libraries/System.Net.Quic/src/Resources/Strings.resx	Adds a new localized string for the new InvalidOperationException message.

[ManickaP]

/ba-g unrelated