test: Add GUI integration tests for external editor with live_gui fixture

Note: Due to process boundaries (GUI runs in subprocess), monkeypatch doesn't
cross to GUI subprocess. Manual verification requires configuring
config.toml in project root with VSCode path.

src/gui_2.py

@@ -129,7 +129,8 @@ class App:
   self.controller._predefined_callbacks['set_ui_screenshot_paths'] = lambda p: setattr(self, 'ui_screenshot_paths', p)
   self.controller._clickable_actions.update({
    'btn_undo': self._handle_undo,
-   'btn_redo': self._handle_redo
+   'btn_redo': self._handle_redo,
+   'btn_open_external_editor': self._open_patch_in_external_editor,
   })
   def simulate_save_preset(name: str):
    from src import models
@@ -2229,6 +2230,7 @@ class App:
    self._patch_error_message = str(e)
 
  def _open_patch_in_external_editor(self) -> None:
+  self._external_editor_clicked = True
   try:
    from src.external_editor import get_default_launcher, create_temp_modified_file
    import os