Add test showing that not all the messages inside a Lite FieldSet wer…

…e made immutable.

I will fix this test in the next CL.

This test runs on both `GeneratedMessage` and `GeneratedMessageLite` variants, so we need to first check if `nested instanceof GeneratedMessageLite`, otherwise on the full-variant test, we get `isMutable()` method found on `GeneratedMessage`.

PiperOrigin-RevId: 671563318

java/core/src/test/java/com/google/protobuf/ParserLiteTest.java

@@ -75,6 +75,19 @@ public void testParseExtensionsLite() throws Exception {
         TestUtilLite.getAllLiteExtensionsSet(), TestUtilLite.getExtensionRegistryLite());
   }
 
+  @Test
+  public void testParseExtensionsLite_extensionIsMutable() throws Exception {
+    TestAllExtensionsLite message =
+        TestAllExtensionsLite.parseFrom(
+            TestUtilLite.getAllLiteExtensionsSet().toByteArray(),
+            TestUtilLite.getExtensionRegistryLite());
+    Object nested = message.getExtension(UnittestLite.optionalNestedMessageExtensionLite);
+    if (nested instanceof GeneratedMessageLite) {
+      // TODO: Make this false.
+      assertThat(((GeneratedMessageLite) nested).isMutable()).isTrue();
+    }
+  }
+
   @Test
   public void testParsePacked() throws Exception {
     assertRoundTripEquals(TestUtil.getPackedSet());