Support nested definitions in GetMessageClassesForFiles

Otherwise downstream callers must explicitly fetch and generate message classes for nested definitions. (For example [here](foxglove/mcap#1321).)

Example:
```
message NestedTypeMessage {
  message NestedType {
      string data = 1;
    }

  NestedType nested = 1;
}
```

Currently `GetMessageClassesForFiles()` will only return `NestedTypeMessage`, but miss `NestedTypeMessage.NestedType` (and any arbitrary level of nested definitions).

This is an unexpected deviation from the comment that says that it will find and resolve all dependencies.

PiperOrigin-RevId: 719854337

python/google/protobuf/internal/message_factory_test.py

@@ -294,6 +294,29 @@ def testExtensionValueInDifferentFile(self):
     self.assertEqual(234, m.Extensions[ext1].setting)
     self.assertEqual(345, m.Extensions[ext2].setting)
 
+  def testNestedDefinition(self):
+    f = descriptor_pb2.FileDescriptorProto(
+        name='google/protobuf/internal/meta_class.proto',
+        package='google.protobuf.python.internal')
+    msg_proto = f.message_type.add(name='Empty')
+    msg_proto.nested_type.add(name='Nested')
+    msg_proto.field.add(name='nested_field',
+                        number=1,
+                        label=descriptor.FieldDescriptor.LABEL_REPEATED,
+                        type=descriptor.FieldDescriptor.TYPE_MESSAGE,
+                        type_name='Nested')
+
+    msg_proto.nested_type[0].nested_type.add(name='DoublyNested')
+    msg_proto.nested_type[0].field.add(name='doubly_nested_field',
+                                       number=2,
+                                       label=descriptor.FieldDescriptor.LABEL_REPEATED,
+                                       type=descriptor.FieldDescriptor.TYPE_MESSAGE,
+                                       type_name='DoublyNested')
+
+    messages = message_factory.GetMessages([f])
+    self.assertIn('google.protobuf.python.internal.Empty.Nested', messages)
+    self.assertIn('google.protobuf.python.internal.Empty.Nested.DoublyNested', messages)
+
   def testDescriptorKeepConcreteClass(self):
     def loadFile():
       f= descriptor_pb2.FileDescriptorProto(
@@ -310,6 +333,8 @@ def loadFile():
 
     messages = loadFile()
     for des, meta_class in messages.items():
+      if des == "google.protobuf.python.internal.Empty.Nested":
+        continue
       message = meta_class()
       nested_des = message.DESCRIPTOR.nested_types_by_name['Nested']
       nested_msg = nested_des._concrete_class()

python/google/protobuf/message_factory.py

@@ -56,6 +56,8 @@ def GetMessageClassesForFiles(files, pool):
   This will find and resolve dependencies, failing if the descriptor
   pool cannot satisfy them.
 
+  This will also recursively find any nested definitions.
+
   Args:
     files: The file names to extract messages from.
     pool: The descriptor pool to find the files including the dependent files.
@@ -69,6 +71,13 @@ def GetMessageClassesForFiles(files, pool):
     for desc in file_desc.message_types_by_name.values():
       result[desc.full_name] = GetMessageClass(desc)
 
+      # Recursively load protos for nested definitions.
+      nested_descriptions = list(desc.nested_types_by_name.values())
+      while nested_descriptions:
+        nested_desc = nested_descriptions.pop()
+        result[nested_desc.full_name] = GetMessageClass(nested_desc)
+        nested_descriptions.extend(nested_desc.nested_types_by_name.values())
+
     # While the extension FieldDescriptors are created by the descriptor pool,
     # the python classes created in the factory need them to be registered
     # explicitly, which is done below.