xds: fix race condition in ext_proc client interceptor halfClose (#12948)

Fixes a race condition in
`ExternalProcessorClientInterceptor$DataPlaneClientCall.halfClose()`
that can cause the call to hang when the external processor stream
completes concurrently.

If the external processor stream completes (marking state as COMPLETED
and setting passThroughMode to true) after `halfClose()` has already
checked `passThroughMode` (seeing false), halfClose() would subsequently
see the state as COMPLETED and return early without invoking
`proceedWithHalfClose()`.

This commit fixes the race by re-checking passThroughMode inside the
`isCompleted()` block in `halfClose()`, ensuring we proceed with
half-close if pass-through mode was enabled late.

Author: kannanjgithub

xds/src/main/java/io/grpc/xds/ExternalProcessorClientInterceptor.java

@@ -860,6 +860,11 @@ public void halfClose() {
       pendingHalfClose.set(true);
 
       if (extProcStreamState.get().isCompleted()) {
+        if (passThroughMode.get()) {
+          if (requestSideClosed.compareAndSet(false, true)) {
+            proceedWithHalfClose();
+          }
+        }
         return;
       }