servlet: fix UndertowTransportTest flakiness by eliminating container deadlocks

[AgraVator]

Background & Problem Statement

UndertowTransportTest.basicStream intermittently failed in CI with:

java.lang.AssertionError: message expected
    at io.grpc.internal.AbstractTransportTest.basicStream(AbstractTransportTest.java:917)

Root Cause

AsyncServletOutputStreamWriter previously assumed that readyAndDrained could only remain true while onWritePossible() was actively executing.

However, when written payloads fit inside the servlet output buffer, outputStream.isReady() remains true after runOrBuffer() finishes, so runOrBuffer() exits leaving readyAndDrained == true.

When Undertow subsequently dispatched WriteListener.onWritePossible() after async I/O write completion:

1. onWritePossible() saw readyAndDrained == true.
2. It called assureReadyAndDrainedTurnsFalse(), which parked the Undertow container worker thread via LockSupport.parkNanos(TimeUnit.MINUTES.toNanos(1)).
3. Because the application thread had already returned from runOrBuffer(), no thread was running to clear readyAndDrained, leaving Undertow blocked for 60 seconds.
4. Meanwhile, AbstractTransportTest.basicStream() timed out after 10 seconds waiting at clientStreamListener.messageQueue.poll(), triggering AssertionError: message expected.

Solution

1. Removed assureReadyAndDrainedTurnsFalse() and LockSupport parking:
   A state of readyAndDrained == true with an empty writeChain and isReady() == true is a valid quiescent state; onWritePossible() verifies the queue is empty and exits cleanly without parking worker threads.
2. Simplified CAS updates in runOrBuffer():
   Updated state cleanup when outputStream becomes unready (isReady() == false), resolving concurrency races caught by Lincheck model checking.
3. Unit Tests:
   Added AsyncServletOutputStreamWriterTest verifying non-blocking execution of onWritePossible().

Fixes #12778