Turn CheckApiAction into a standard-ish epp endpoint

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125335634

java/google/registry/flows/EppRequestHandler.java

@@ -15,6 +15,7 @@
 
 package google.registry.flows;
 
+import static google.registry.flows.EppXmlTransformer.marshalWithLenientRetry;
 import static java.nio.charset.StandardCharsets.UTF_8;
 import static javax.servlet.http.HttpServletResponse.SC_BAD_REQUEST;
 import static javax.servlet.http.HttpServletResponse.SC_OK;
@@ -48,13 +49,15 @@ public class EppRequestHandler {
       byte[] inputXmlBytes) {
     try {
       response.setPayload(new String(
-          eppController.handleEppCommand(
-              sessionMetadata,
-              credentials,
-              eppRequestSource,
-              isDryRun,
-              isSuperuser,
-              inputXmlBytes), UTF_8));
+          marshalWithLenientRetry(
+              eppController.handleEppCommand(
+                  sessionMetadata,
+                  credentials,
+                  eppRequestSource,
+                  isDryRun,
+                  isSuperuser,
+                  inputXmlBytes)),
+          UTF_8));
       response.setContentType(APPLICATION_EPP_XML);
       // Note that we always return 200 (OK) even if the EppController returns an error response.
       // This is because returning an non-OK HTTP status code will cause the proxy server to