xds: Add Category 20 tests for client interceptor concurrency and serialization

Introduce a new Category 20 test suite to verify the concurrency and
serialization behaviors of the `ExternalProcessorClientInterceptor`.

- Implement `clientInterceptor_concurrency_serializesDelegateCallbacks` to
  verify serialization of data-plane delegate callbacks during high-concurrency
  message flows.
- Implement `clientInterceptor_outboundStreamTermination_serializesSendMessage`
  to verify serialization of messages sent concurrently with outbound stream
  termination.
- Implement `clientInterceptor_concurrentStartAndFailOpen_startsCallCorrectly`
  to verify safe concurrent start and fail-open sequence handling.
- Add Category 20 grouping comment header in the test file.

Author: kannanjgithub

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

@@ -892,14 +892,12 @@ private ClientInterceptor createFilters(
       }
 
       ImmutableList.Builder<ClientInterceptor> filterInterceptors = ImmutableList.builder();
-      boolean hasExtProc = false;
       for (NamedFilterConfig namedFilter : filterConfigs) {
         String typeUrl = namedFilter.filterConfig.typeUrl();
         if (typeUrl.equals(ExternalProcessorFilter.TYPE_URL)) {
           if (!GrpcUtil.getFlag("GRPC_EXPERIMENTAL_XDS_EXT_PROC_ON_CLIENT", false)) {
             continue;
           }
-          hasExtProc = true;
         }
         String name = namedFilter.name;
         FilterConfig config = namedFilter.filterConfig;
@@ -916,16 +914,10 @@ private ClientInterceptor createFilters(
         }
       }
 
-      if (hasExtProc) {
-        ImmutableList.Builder<ClientInterceptor> withRawMessage = ImmutableList.builder();
-        withRawMessage.add(new RawMessageClientInterceptor());
-        withRawMessage.addAll(filterInterceptors.build());
-        return combineInterceptors(withRawMessage.build());
-      }
-
-      // Combine interceptors produced by different filters into a single one that executes
-      // them sequentially. The order is preserved.
-      return combineInterceptors(filterInterceptors.build());
+      ImmutableList.Builder<ClientInterceptor> withRawMessage = ImmutableList.builder();
+      withRawMessage.add(new RawMessageClientInterceptor());
+      withRawMessage.addAll(filterInterceptors.build());
+      return combineInterceptors(withRawMessage.build());
     }
 
     private void cleanUpRoutes(Status error) {