Fix ext_proc mock response handling in clientInterceptor_contextPropagatedToStartCall

In ExternalProcessorClientInterceptorTest.clientInterceptor_contextPropagatedToStartCall(),
the mock ext_proc service blindly returned a ProcessingResponse with setRequestHeaders
for every incoming ProcessingRequest. When the downstream RPC finished and sent back response
headers, the interceptor sent a ProcessingRequest with responseHeaders. The mock's
blind requestHeaders reply caused the interceptor to detect an out-of-order protocol error,
triggering internalOnError() which called delayedCall.cancel() on the executor thread.

This async cancellation on the executor thread raced with the test code's cleanup
proxyCall.cancel() on the test runner thread, causing TSAN to report a data race on
the non-volatile field ClientCallImpl.cancelCalled.

This commit fixes the test mock to check the request type (hasRequestHeaders() vs.
hasResponseHeaders()) before responding, matching the behavior in
clientInterceptor_contextPropagatedToListenerCallbacks().

Note: The unsynchronized cancellation between the application thread and an interceptor's
cancellation on an executor thread, while technically a data race from TSAN's perspective
due to ClientCallImpl.cancelCalled not being volatile, is not consequential in practice
because the underlying stream.cancel() and CancellationHandler.tearDown() are thread-safe
and idempotent. Therefore, modifying cancelCalled in ClientCallImpl is not necessary.

Author: kannanjgithub

xds/src/test/java/io/grpc/xds/ExternalProcessorClientInterceptorTest.java

@@ -14309,9 +14309,15 @@ public StreamObserver<ProcessingRequest> process(
             return new StreamObserver<ProcessingRequest>() {
               @Override
               public void onNext(ProcessingRequest request) {
-                responseObserver.onNext(ProcessingResponse.newBuilder()
-                    .setRequestHeaders(HeadersResponse.newBuilder().build())
-                    .build());
+                if (request.hasRequestHeaders()) {
+                  responseObserver.onNext(ProcessingResponse.newBuilder()
+                      .setRequestHeaders(HeadersResponse.newBuilder().build())
+                      .build());
+                } else if (request.hasResponseHeaders()) {
+                  responseObserver.onNext(ProcessingResponse.newBuilder()
+                      .setResponseHeaders(HeadersResponse.newBuilder().build())
+                      .build());
+                }
               }
 
               @Override