[commands] Fix app command children not being copied in HybridGroup

This degenerate case is only triggered inside cogs when using the
app_command property to define commands, such as this:

    class X(commands.Cog):
        @commands.hybrid_group()
        async def foo(self, ctx):
            ...

        @foo.app_command.command()
        async def bar(self, interaction):
            ...

discord/ext/commands/cog.py

@@ -289,17 +289,19 @@ class Cog(metaclass=CogMeta):
                 # Update our parent's reference to our self
                 parent.remove_command(command.name)  # type: ignore
                 parent.add_command(command)  # type: ignore
-            elif self.__cog_app_commands_group__:
-                if hasattr(command, '__commands_is_hybrid__') and command.parent is None:
-                    parent = self.__cog_app_commands_group__
-                    app_command: Optional[Union[app_commands.Group, app_commands.Command[Self, ..., Any]]] = getattr(
-                        command, 'app_command', None
-                    )
-                    if app_command:
-                        app_command = app_command._copy_with(parent=parent, binding=self)
+
+            if hasattr(command, '__commands_is_hybrid__') and parent is None:
+                app_command: Optional[Union[app_commands.Group, app_commands.Command[Self, ..., Any]]] = getattr(
+                    command, 'app_command', None
+                )
+                if app_command:
+                    group_parent = self.__cog_app_commands_group__
+                    app_command = app_command._copy_with(parent=group_parent, binding=self)
+                    # The type checker does not see the app_command attribute even though it exists
+                    command.app_command = app_command  # type: ignore
+
+                    if self.__cog_app_commands_group__:
                         children.append(app_command)
-                        # The type checker does not see the app_command attribute even though it exists
-                        command.app_command = app_command  # type: ignore
 
         for command in cls.__cog_app_commands__:
             copy = command._copy_with(parent=self.__cog_app_commands_group__, binding=self)

discord/ext/commands/hybrid.py

@@ -565,6 +565,19 @@ class HybridGroup(Group[CogT, P, T]):
         copy.fallback = self.fallback
         return copy
 
+    def _update_copy(self, kwargs: Dict[str, Any]) -> Self:
+        copy = super()._update_copy(kwargs)
+        # This is only really called inside CogMeta
+        # I want to ensure that the children of the app command are copied
+        # For example, if someone defines an app command using the app_command property
+        # while inside the cog. This copy is not propagated in normal means.
+        # For some reason doing this copy above will lead to duplicates.
+        if copy.app_command and self.app_command:
+            # This is a very lazy copy because the CogMeta will properly copy it
+            # with bindings and all later
+            copy.app_command._children = self.app_command._children.copy()
+        return copy
+
     def autocomplete(
         self, name: str
     ) -> Callable[[AutocompleteCallback[CogT, ChoiceT]], AutocompleteCallback[CogT, ChoiceT]]: