Fixes to SlashCommand.mention using qualified_id (#1672)

* Implement qualified_id to fix command.mention

* Update discord/commands/core.py

Co-authored-by: BobDotCom <71356958+BobDotCom@users.noreply.github.com>

Co-authored-by: BobDotCom <71356958+BobDotCom@users.noreply.github.com>

discord/commands/core.py

@@ -582,6 +582,17 @@ def qualified_name(self) -> str:
         else:
             return self.name
 
+    @property
+    def qualified_id(self) -> int:
+        """:class:`int`: Retrieves the fully qualified command ID.
+
+        This is the root parent ID. For example, in ``/one two three``
+        the qualified ID would return ``one.id``.
+        """
+        if self.id is None:
+            return self.parent.qualified_id
+        return self.id
+
     def to_dict(self) -> dict[str, Any]:
         raise NotImplementedError
 
@@ -821,7 +832,7 @@ def is_subcommand(self) -> bool:
 
     @property
     def mention(self) -> str:
-        return f"</{self.qualified_name}:{self.id}>"
+        return f"</{self.qualified_name}:{self.qualified_id}>"
 
     def to_dict(self) -> dict:
         as_dict = {