Detach entities loaded by loadSingleton() (#1184)

* Detach entities loaded by loadSingleton()

* Reformatted

core/src/main/java/google/registry/persistence/transaction/JpaTransactionManagerImpl.java

@@ -504,7 +504,7 @@ public class JpaTransactionManagerImpl implements JpaTransactionManager {
         elements.size() <= 1,
         "Expected at most one entity of type %s, found at least two",
         clazz.getSimpleName());
-    return elements.stream().findFirst();
+    return elements.stream().findFirst().map(this::detach);
   }
 
   private int internalDelete(VKey<?> key) {

core/src/test/java/google/registry/persistence/transaction/JpaTransactionManagerImplTest.java

@@ -508,6 +508,19 @@ class JpaTransactionManagerImplTest {
     assertThat(persisted).containsExactlyElementsIn(moreEntities);
   }
 
+  @Test
+  void loadSingleton_detaches() {
+    jpaTm().transact(() -> jpaTm().insert(theEntity));
+    jpaTm()
+        .transact(
+            () ->
+                assertThat(
+                    jpaTm()
+                        .getEntityManager()
+                        .contains(jpaTm().loadSingleton(TestEntity.class).get())))
+        .isFalse();
+  }
+
   @Test
   void delete_succeeds() {
     jpaTm().transact(() -> jpaTm().insert(theEntity));