Suppress EC flow in BeginErrorReadLine and fix diagnostic log glob

- Add ExecutionContext.SuppressFlow() around process.BeginErrorReadLine()
  to prevent the Process's internal async stderr reader from capturing
  the caller's execution context. This avoids potential interference
  between the background reader thread and the caller's async tracking
  (e.g., xUnit test runner state), which was correlated with intermittent
  test host hangs on .NET 9+.

- Fix CI artifact upload glob: change artifacts/bin/*Tests/*/xunit-runtest-diag.log
  to **/xunit-runtest-diag.log so the instrumented xUnit diagnostic log is
  actually captured when the test host hangs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ericstj

.github/workflows/ci-build-test.yml

@@ -86,7 +86,7 @@ jobs:
         name: testresults-${{ matrix.os }}-${{ matrix.configuration }}
         path: |
           artifacts/testresults/**
-          artifacts/bin/*Tests/*/xunit-runtest-diag.log
+          **/xunit-runtest-diag.log
 
   publish-coverage:
     if: github.actor != 'dependabot[bot]'

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -203,7 +203,12 @@ public async Task<ITransport> ConnectAsync(CancellationToken cancellationToken =
 
             LogTransportProcessStarted(logger, endpointName, process.Id);
 
-            process.BeginErrorReadLine();
+            // Suppress ExecutionContext flow so the Process's internal async
+            // stderr reader thread doesn't capture the caller's context.
+            using (ExecutionContext.SuppressFlow())
+            {
+                process.BeginErrorReadLine();
+            }
 
             return new StdioClientSessionTransport(_options, process, endpointName, stderrRollingLog, errorHandler, _loggerFactory);
         }