xds: improve test coverage for ext_proc client interceptor

Add comprehensive tests for directional and configuration order validation
in ExternalProcessorClientInterceptor. Fix order check so clientEndOfStreamReceived
is evaluated before clientHeadersReceived (and trailers/EOS before headers).

TAG=agy
CONV=581f86a6-097d-44a2-9d6e-6f9d2e83cbc0

Author: kannanjgithub

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

@@ -573,18 +573,18 @@ public void onNext(ProcessingResponse response) {
                     .asRuntimeException());
                 return;
               }
-              if (clientHeadersReceived) {
+              if (clientEndOfStreamReceived) {
                 internalOnError(Status.UNAVAILABLE
                     .withDescription("Protocol error: received response out of order. "
-                        + "Duplicate request_headers response.")
+                        + "Received request_headers after end-of-stream in "
+                        + "client-to-server direction.")
                     .asRuntimeException());
                 return;
               }
-              if (clientEndOfStreamReceived) {
+              if (clientHeadersReceived) {
                 internalOnError(Status.UNAVAILABLE
                     .withDescription("Protocol error: received response out of order. "
-                        + "Received request_headers after end-of-stream in "
-                        + "client-to-server direction.")
+                        + "Duplicate request_headers response.")
                     .asRuntimeException());
                 return;
               }
@@ -604,17 +604,17 @@ public void onNext(ProcessingResponse response) {
                     .asRuntimeException());
                 return;
               }
-              if (serverHeadersReceived) {
+              if (serverTrailersReceived || serverEndOfStreamReceived) {
                 internalOnError(Status.UNAVAILABLE
                     .withDescription("Protocol error: received response out of order. "
-                        + "Duplicate response_headers response.")
+                        + "Received response_headers after trailers or end-of-stream.")
                     .asRuntimeException());
                 return;
               }
-              if (serverTrailersReceived || serverEndOfStreamReceived) {
+              if (serverHeadersReceived) {
                 internalOnError(Status.UNAVAILABLE
                     .withDescription("Protocol error: received response out of order. "
-                        + "Received response_headers after trailers or end-of-stream.")
+                        + "Duplicate response_headers response.")
                     .asRuntimeException());
                 return;
               }