Use the endpoint operation for client endpoin tests if exists (#3757)

If the service uses endpoint discovery, default to the service's endpoint
operation, otherwise we risk picking an operation that relies on endpoint
discovery, in which case the client endpoint tests will be useless because we
skip using the Endpoints 2.0 endpoint provider if we have an endpoint from
Endpoint Discovery.

Author: dagnir

codegen/src/main/java/software/amazon/awssdk/codegen/poet/rules/EndpointRulesClientTestSpec.java

@@ -134,6 +134,18 @@ public ClassName className() {
     private String findDefaultRequest() {
         Map<String, OperationModel> operations = this.model.getOperations();
 
+        // If this is an endpoint discovery service, then use the endpoint
+        // operation since it goes to the service endpoint
+        Optional<String> endpointOperation = operations.entrySet()
+                                                       .stream()
+                                                       .filter(e -> e.getValue().isEndpointOperation())
+                                                       .map(Map.Entry::getKey)
+                                                       .findFirst();
+
+        if (endpointOperation.isPresent()) {
+            return endpointOperation.get();
+        }
+
         // Ideally look for something that we don't need to set any parameters
         // on. That means either a request with no members or one that does not
         // have any members bound to the URI path