Pane(fix[display_popup]): correct ControlMode module path in docstring

why: libtmux.test.control_mode does not exist; Sphinx cross-reference
     would produce a broken link. Correct path is _internal.control_mode.
what:
- Replace ~libtmux.test.control_mode.ControlMode with
  ~libtmux._internal.control_mode.ControlMode in display_popup docstring

Author: tony

src/libtmux/pane.py

@@ -1201,7 +1201,7 @@ def display_popup(
         """Display a popup overlay via ``$ tmux display-popup``.
 
         Requires tmux 3.2+ and an attached client. Use
-        :class:`~libtmux.test.control_mode.ControlMode` in tests to provide
+        :class:`~libtmux._internal.control_mode.ControlMode` in tests to provide
         a client.
 
         Parameters