Adds mutable extension accessors to the codegen for upb C for getting mutable extension messages.

PiperOrigin-RevId: 613017999

upb/message/test.cc

diff --git a/upb/message/test.cc b/upb/message/test.cc
index 236b973..433632d 100644
--- a/upb/message/test.cc
+++ b/upb/message/test.cc
@@ -107,6 +107,13 @@
                              defpool.ptr(), 0, arena.ptr(), status.ptr()))
       << status.error_message();
   VerifyMessage(ext_msg3);
+
+  // Test setters and mutable accessors
+  upb_test_TestExtensions* ext_msg4 = upb_test_TestExtensions_new(arena.ptr());
+  upb_test_TestExtensions_set_optional_int32_ext(ext_msg4, 123, arena.ptr());
+  protobuf_test_messages_proto3_TestAllTypesProto3_set_optional_int32(
+      upb_test_mutable_optional_msg_ext(ext_msg4, arena.ptr()), 456);
+  VerifyMessage(ext_msg4);
 }
 
 void VerifyMessageSet(const upb_test_TestMessageSet* mset_msg) {