servlet: fix TomcatTransportTest detects write when not ready

[themechbro]

Fixes #12723

What was happening

In AsyncServletOutputStreamWriter#runOrBuffer, the application thread was blindly trusting the cached readyAndDrained boolean state. Under certain asynchronous conditions (e.g., TCP window shrinking), Tomcat's internal buffer would fill up, but readyAndDrained was still true. Bypassing the live isReady() check caused the container to throw an IllegalStateException when actionItem.run() was executed.

The Fix

This PR updates the primary execution path in runOrBuffer to explicitly evaluate isReady.getAsBoolean() alongside the cached state.

If the cached state is true but the container is actually not ready, the thread gracefully drops into the else block, buffers the action into the writeChain, and attempts to update readyAndDrained to false.

Handling Concurrent CAS Failures

By forcing threads into the else block when isReady() evaluates to false, we introduce a safe race condition: multiple threads might try to execute writeState.compareAndSet(true, false) simultaneously.

To prevent the losing threads from crashing on the subsequent checkState, an explicit check for the initial state (if (curState.readyAndDrained)) was added inside the CAS failure block. If a thread started as true but failed the CAS, it simply means another concurrent thread already safely toggled the state to false. The thread performs a safe no-op and exits, allowing Tomcat's onWritePossible() to eventually drain the queue.

Passed local testing via:
./gradlew :grpc-servlet:tomcat9Test --tests "*TomcatTransportTest*" -PskipAndroid=true -PskipCodegen=true