[controller] Stub normalizeSchemaForMigration in parent admin test mock

VeniceParentHelixAdmin#createStore and #addValueSchema now route value
schemas through VeniceHelixAdmin#normalizeSchemaForMigration. The internal
admin is a Mockito mock in TestVeniceParentHelixAdmin, so the unstubbed
method returned null and blanked out the value schema, producing NPEs
during admin-message serialization and strict schema parsing.

Mirror production's non-migration passthrough behavior by stubbing the
method to return its schema argument verbatim.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: xunyin8

services/venice-controller/src/test/java/com/linkedin/venice/controller/AbstractTestVeniceParentHelixAdmin.java

@@ -98,6 +98,11 @@ public void setupInternalMocks() {
     doReturn(topicManager).when(internalAdmin).getTopicManager();
     SchemaEntry mockEntry = new SchemaEntry(0, TEST_SCHEMA);
     doReturn(mockEntry).when(internalAdmin).getKeySchema(anyString(), anyString());
+    // Outside a store-migration context, schema normalization is a passthrough. Mirror that here so
+    // the mocked internal admin doesn't return null and blank out the value schema during createStore/
+    // addValueSchema.
+    when(internalAdmin.normalizeSchemaForMigration(anyString(), anyString(), any()))
+        .thenAnswer(invocation -> invocation.getArgument(2));
 
     zkClient = mock(ZkClient.class);
     doReturn(zkClient).when(internalAdmin).getZkClient();