Skip flaky test on Mono where stderr callbacks are unsupported (#980)

* Initial plan

* Fix race condition in StdioClientSessionTransport stderr handling

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Simplify WaitForExit logic as suggested in code review

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Skip flaky test on Mono and revert incorrect fix

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

tests/ModelContextProtocol.Tests/Transport/StdioClientTransportTests.cs

@@ -22,8 +22,7 @@ public async Task CreateAsync_ValidProcessInvalidServer_Throws()
         await Assert.ThrowsAsync<IOException>(() => McpClient.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
     }
 
-    // [Fact(Skip = "Platform not supported by this test.", SkipUnless = nameof(IsStdErrCallbackSupported))]
-    [Fact]
+    [Fact(Skip = "Platform not supported by this test.", SkipUnless = nameof(IsStdErrCallbackSupported))]
     public async Task CreateAsync_ValidProcessInvalidServer_StdErrCallbackInvoked()
     {
         string id = Guid.NewGuid().ToString("N");