TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 5: refactor(gui_2): migrate L5920 render_external_editor_panel config to Result[T] (Phase 5)

Extract _render_external_editor_panel_config_result helper from the external
editor config rendering try/except in render_external_editor_panel. Legacy
wrapper drains errors to app._last_request_errors per FR-BC-4
event-handler pattern.

[pre-audit] L5920 INTERNAL_BROAD_CATCH
[post-audit] V count: 5 -> 4 (L5920 removed)

tests/test_gui_2_result.py

@@ -1185,3 +1185,44 @@ def test_phase_5_l5786_render_text_viewer_window_ced_result_failure():
  err = result.errors[0]
  assert err.source == "gui_2._render_text_viewer_window_ced_result"
  assert "ced set_text failed" in err.message
+
+
+def test_phase_5_l5920_render_external_editor_panel_config_result_success():
+ """
+ L5920 _render_external_editor_panel_config_result returns Result.ok=True on success.
+
+ The helper wraps the external editor config rendering try/except in
+ render_external_editor_panel. On success, returns Result(data=True).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ app.config = {"tools": {"text_editors": {}, "default_editor": ""}}
+ mock_launcher = MagicMock()
+ mock_launcher.config.editors        = {}
+ mock_launcher.config.default_editor = None
+ with patch("src.external_editor.get_default_launcher", return_value=mock_launcher), \
+   patch.object(gui_2, "imgui", new=MagicMock()):
+  result = gui_2._render_external_editor_panel_config_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is True
+
+
+def test_phase_5_l5920_render_external_editor_panel_config_result_failure():
+ """
+ L5920 _render_external_editor_panel_config_result returns Result.ok=False with ErrorInfo on failure.
+
+ When the external editor config rendering raises, the helper converts the
+ exception to ErrorInfo and returns Result(data=False, errors=[ErrorInfo]).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ app.config = {"tools": {"text_editors": {}, "default_editor": ""}}
+ with patch("src.external_editor.get_default_launcher", side_effect=RuntimeError("ext editor config blew up")):
+  result = gui_2._render_external_editor_panel_config_result(app)
+ assert not result.ok, f"Expected ok=False on failure, got data: {result.data}"
+ assert result.errors, "Expected at least one error on failure"
+ err = result.errors[0]
+ assert err.source == "gui_2._render_external_editor_panel_config_result"
+ assert "ext editor config blew up" in err.message