StdioServerTransport does not reliably close on linux, which causes a
threadpool thread to be blocked on read.  This can lead to threadpool
starvation and application hangs.  Since the tests in question do not
require stdio transport, switch to using StreamServerTransport with null
streams to avoid this issue.

TEMPORARY diagnostic instrumentation to capture what happens during the
intermittent xUnit v3 RunTest hang where finished TCS is never signaled.

Changes:
- Instrumented xunit.v3.core.dll with trace points in TestRunner.RunTest:
  before/after EC.Run, runTest entry, pre-try completion, test invocation,
  and finished signaling. Also moves pre-try code inside try block as a
  structural fix.
- DiagnosticExceptionTracing.cs module initializer hooking
  UnhandledException and UnobservedTaskException for additional context.
- Both test csproj files copy instrumented DLLs post-build via
  ReplaceXunitWithInstrumented target.
- CI workflow collects xunit-runtest-diag.log as test artifact.

All instrumentation writes to xunit-runtest-diag.log and stderr.
Remove once the root cause is identified.

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

The async chain from ConnectAsync through initializeTask/processingTask
can become GC-unreachable when the server process exits before responding.

Task.Delay creates a System.Threading.Timer that is directly rooted in
the runtime's timer queue, providing a simple GC root chain:
Timer Queue -> Timer -> Task.Delay -> WhenAny -> ConnectAsync.

This ensures ConnectAsync always resumes within the initialization
timeout, even if intermediate state machines are collected.

Use a dedicated CTS for the delay (not the caller's cancellationToken)
so external cancellation flows through initializeTask via
initializationCts, preserving the expected OperationCanceledException
propagation path.

Only call FailPendingRequests when processingTask wins (server exited).
When delayTask wins, let the CancelAfter timer cancel initializationCts
naturally, producing TimeoutException through the existing OCE catch
block.

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

xUnit v3's TestRunner.RunTest wraps test execution in an async void
callback (required by ExecutionContext.Run's ContextCallback signature).
When the test's async continuation chain forms a cycle with no external
GC root (no I/O, no timers), the entire object graph is collectible.
In Release mode on Ubuntu, the JIT's aggressive optimizations make this
reproducible — the async void state machine gets collected, the test
never completes, and the test host hangs.

The fix extracts the async void body into an async Task method and
holds a reference to the returned Task, keeping the entire async chain
GC-rooted for the lifetime of RunTest.

Also reverts prior ConnectAsync/McpSessionHandler workarounds that
were attempting to address this from the product side.

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

GetUnexpectedExitExceptionAsync used a linked CancellationTokenSource
that included the caller's token (_shutdownCts.Token). When
ReadMessagesAsync and DisposeAsync race to call CleanupAsync, the
loser calls CancelShutdown() which cancels _shutdownCts. This
prematurely aborted WaitForExitAsync in the winner's cleanup path,
preventing stderr pipe buffers from being fully drained. The
ErrorDataReceived callback would never fire for lines still in the
pipe, causing CreateAsync_ValidProcessInvalidServer_StdErrCallbackInvoked
to fail intermittently.

Fix: use a standalone timeout CTS instead of linking to the caller's
token. The process is already dead at this point—we only need a
timeout to bound the pipe drain, not external cancellation.

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

…-sdk into fix1448

Extract WaitForProcessExitAsync to ensure ErrorDataReceived events are
fully drained before the handler is detached. Fixes a race where
HasExited returns false while the process is exiting (stdout closed but
not yet reaped), causing GetUnexpectedExitExceptionAsync to skip the
drain and the handler to be detached before callbacks arrive.

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