test(conftest): Add VSCode config to live_gui fixture, update external editor tests

- conftest.py: Include tools.text_editors.vscode in live_gui workspace config
- gui_2.py: Add btn_open_external_editor to _clickable_actions
- test_external_editor_gui.py: Tests for external editor GUI integration

Note: Due to process boundaries (GUI runs in subprocess), full VSCode launch
verification requires manual testing. The test infrastructure verifies config,
command format, and button wiring. Manual verification recommended.

tests/conftest.py

@@ -213,6 +213,15 @@ def live_gui() -> Generator[tuple[subprocess.Popen, str], None, None]:
   'paths': {
    'logs_dir': str((temp_workspace / "logs").absolute()),
    'scripts_dir': str((temp_workspace / "scripts" / "generated").absolute())
+  },
+  'tools': {
+   'text_editors': {
+    'vscode': {
+     'path': 'C:\\apps\\Microsoft VS Code\\Code.exe',
+     'diff_args': ['--new-window', '--diff']
+    }
+   },
+   'default_editor': {'default_editor': 'vscode'}
   }
  }
  import tomli_w