xds: enforce trailer mode SEND for GRPC response body mode in ext_proc client

- Enforce that responseTrailerMode must be SEND if responseBodyMode is GRPC
  in ExternalProcessorFilter configuration validation.
- Refactor ExternalProcessorClientInterceptor.triggerCloseHandshake to cleanly
  close the ext_proc stream when trailers are skipped, removing the legacy
  empty body end-of-stream signal.
- Update unit tests in ExternalProcessorClientInterceptorTest to configure
  responseTrailerMode = SEND when using responseBodyMode = GRPC and perform
  the trailers handshake.
- Rename givenExtProcRespondsWithEndOfStream_whenAppHasNotHalfClosed_thenClosePropagated
  to givenResponseTrailerModeSend_whenDataPlaneCloses_thenTrailersHandshakeCompleted.

Author: kannanjgithub

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

@@ -1370,14 +1370,8 @@ private void triggerCloseHandshake() {
                 .build())
             .build());
       } else {
-        // Send EOS signal via empty body
-        dataPlaneClientCall.sendToExtProc(ProcessingRequest.newBuilder()
-            .setResponseBody(HttpBody.newBuilder()
-                .setEndOfStreamWithoutMessage(true)
-                .build())
-            .build());
-        
         proceedWithClose();
+        dataPlaneClientCall.closeExtProcStream();
       }
     }
 

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

@@ -193,9 +193,9 @@ private static ConfigOrError<Optional<GrpcServiceConfig>> parseAndValidate(
     }
 
     if (mode.getResponseBodyMode() == ProcessingMode.BodySendMode.GRPC
-        && mode.getResponseHeaderMode() != ProcessingMode.HeaderSendMode.SEND) {
-      return ConfigOrError.fromError("Invalid response_header_mode: " + mode.getResponseHeaderMode()
-          + ". response_header_mode must be SEND if response_body_mode is GRPC.");
+        && mode.getResponseTrailerMode() != ProcessingMode.HeaderSendMode.SEND) {
+      return ConfigOrError.fromError("Invalid response_trailer_mode: " + mode.getResponseTrailerMode()
+          + ". response_trailer_mode must be SEND if response_body_mode is GRPC.");
     }
 
     try {