Surface test host crashes during protocol negotiation

[nohwnd]

Follow-up to #16274. The serializer bug itself is fixed in 18.9 and backported to 18.8.1 in #16281, this PR is about diagnosability, so the next time the test host dies during negotiation the user sees why instead of a 90 second timeout.

When the test host crashes while negotiating the protocol version (in #16274 it could not deserialize the version check message because reflection based serialization was disabled) two things hid the cause. The test host could not send the error back, because the protocol is not negotiated yet and serializing the response hits the same failure, so the error only reached the diag log. And CheckVersionWithTestHost only waited on the negotiation event, it ignored the test host exit, so it blocked for the whole connection timeout and then reported a generic "waiting for response timed out after 90 seconds" message.

Now the test host writes the unrecoverable negotiation error to its standard error, which the runner already captures from the process output. CheckVersionWithTestHost waits for the negotiation response or for the test host to exit, whichever comes first, and when the host exits it throws with the captured standard error instead of the timeout message.

Added unit tests for both sides: the test host writes a failed version check to standard error, and the runner surfaces the captured error and stops waiting when the host exits during negotiation.

[Copilot]

Pull request overview

This PR improves diagnosability when the testhost crashes during protocol negotiation (before the protocol is established), ensuring the runner surfaces the underlying error promptly instead of waiting out the full connection timeout and reporting a generic timeout.

Changes:

• Testhost (TestRequestHandler) now writes unrecoverable version-check processing failures to standard error so the runner can capture and display the root cause.
• Runner (TestRequestSender.CheckVersionWithTestHost) now waits for either protocol negotiation or testhost exit, and throws immediately with captured standard error when the host exits during negotiation.
• Adds unit tests covering both: writing negotiation failures to stderr (testhost side) and surfacing stderr on early exit (runner side).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
test/Microsoft.TestPlatform.CrossPlatEngine.UnitTests/EventHandlers/TestRequestHandlerTests.cs	Adds a unit test and a testable override to observe standard-error output from version-check failures.
test/Microsoft.TestPlatform.CommunicationUtilities.UnitTests/TestRequestSenderTests.cs	Adds a unit test ensuring early testhost exit during negotiation throws with captured stderr (not timeout).
src/Microsoft.TestPlatform.CrossPlatEngine/EventHandlers/TestRequestHandler.cs	Writes unrecoverable version-check negotiation failures to standard error via a test-overridable method.
src/Microsoft.TestPlatform.CommunicationUtilities/TestRequestSender.cs	Waits on negotiation-or-exit and throws with captured stderr when the testhost exits during negotiation.

[Evangelink]

Reviewed the negotiation, host-exit, timeout, and stderr-capture paths, including concurrent response/exit ordering and compatibility with older hosts. The implementation is sound.