Fix Flaky HTTP/3 Test: POST_ClientCancellationUpload_RequestAbortRaised

[PreethikaSelvam]

Summary

Fixes intermittent failures in Http3RequestTests.POST_ClientCancellationUpload_RequestAbortRaised caused by timing-dependent behavior when reading the request body after client cancellation in HTTP/3.

Validation / Investigation

The failing test intermittently reported:
Assert.Throws() Failure: No exception was thrown Expected: typeof(System.IO.IOException)

To investigate the issue, the test was executed in a local stress run of 1000 iterations, during which the failure was successfully reproduced. Analysis of both local and CI failures confirmed that:

• Client cancellation was successfully triggered.
• The server observed the RequestAborted notification.
• HTTP/3 stream reset (H3_REQUEST_CANCELLED) was received.
• The request completed with HTTP 499.

The flakiness was caused by the final request body read after cancellation. Depending on timing, two valid outcomes were observed:

ReadAsync() throws:
IOException("The client reset the request stream.")

ReadAsync() completes successfully and returns:
0
indicating EOF.

The existing test only accepted the exception path, causing intermittent failures whenever EOF was observed instead.

Validation

• Successfully reproduced the failure locally using a 1000-iteration stress run.
• Confirmed that RequestAborted was raised in both passing and failing runs.
• Confirmed that the HTTP/3 cancellation path remained unchanged.
• Verified that the updated test handles both valid post-cancellation outcomes (IOException or EOF).
• Test-only change with no product code modifications.

Changes

• Replaced the strict Assert.ThrowsAsync() validation with logic that accepts both valid post-cancellation outcomes: an IOException when the stream reset is observed or a successful read returning 0 bytes (EOF).
• Updated the final request-body read validation to handle timing-dependent HTTP/3 transport behavior instead of assuming cancellation always surfaces as an exception.
• Changed TaskCompletionSource to TaskCompletionSource<Task> to correctly represent the return type of ReadAsync() and enable validation of the returned byte count.
• Preserved existing cancellation verification by continuing to validate that the server observes the RequestAborted notification before evaluating the final read result.
• Stabilized the test by removing reliance on a transport-specific timing assumption while maintaining validation of client cancellation behavior.

Fixes #67518 & #58802

[dotnet-policy-service]

Thanks for your PR, @PreethikaSelvam. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.