xds: Fix concurrent StreamObserver usage in ExternalProcessorClientInterceptorTest (grpc#12915)

Synchronize calls to the gRPC StreamObserver (responseObserver) inside
mock ExternalProcessor implementations. The mock servers in several
tests spawn background threads to handle requests, but they invoke
responseObserver.onNext(), onCompleted(), and onError() concurrently
without synchronization. Since standard StreamObservers are not
thread-safe, this leads to ThreadSanitizer-reported data races in gRPC
internals (such as ServerCallImpl).

This change wraps all asynchronous/background invocations of
responseObserver methods inside synchronized blocks on the observer
instance to ensure serialized execution.

Flagged by TSAN in b/533413322

Author: kannanjgithub