TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 5: refactor(gui_2): migrate L1393 _open_patch_in_external_editor to Result[T] (Phase 5)

Extract _open_patch_in_external_editor_result helper from the external editor
launch try/except in App._open_patch_in_external_editor. Legacy wrapper
drains errors to app._last_request_errors per FR-BC-4 event-handler pattern.

[pre-audit] L1393 INTERNAL_BROAD_CATCH
[post-audit] V count: 11 -> 10 (L1393 removed)

tests/test_gui_2_result.py

@@ -923,3 +923,55 @@ def test_phase_5_l1367_apply_pending_patch_result_failure():
  err = result.errors[0]
  assert err.source == "gui_2._apply_pending_patch_result"
  assert "patch blew up" in err.message
+
+
+def test_phase_5_l1393_open_patch_in_external_editor_result_success():
+ """
+ L1393 _open_patch_in_external_editor_result returns Result.ok=True on success.
+
+ The helper wraps the external editor launch try/except in
+ App._open_patch_in_external_editor. On success (launcher.launch_diff
+ returns a process), returns Result(data=True).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ app._pending_patch_files = ["/proj/foo/src/foo.py"]
+ app._pending_patch_text   = "--- a/foo.py\n+++ b/foo.py\n"
+ mock_editor      = MagicMock(name="mock_editor")
+ mock_launcher    = MagicMock(name="mock_launcher")
+ mock_launcher.config.get_default.return_value = mock_editor
+ mock_process     = MagicMock(name="mock_process")
+ mock_launcher.launch_diff.return_value         = mock_process
+ with patch("os.path.exists", return_value=True), \
+   patch("src.external_editor.get_default_launcher", return_value=mock_launcher), \
+   patch("src.external_editor.create_temp_modified_file", return_value="/tmp/patch_temp.py"):
+  result = gui_2._open_patch_in_external_editor_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is True
+ assert app._vscode_diff_process is mock_process
+
+
+def test_phase_5_l1393_open_patch_in_external_editor_result_failure():
+ """
+ L1393 _open_patch_in_external_editor_result returns Result.ok=False with ErrorInfo on failure.
+
+ When the external editor launch 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._pending_patch_files = ["/proj/foo/src/foo.py"]
+ app._pending_patch_text   = "--- a/foo.py\n+++ b/foo.py\n"
+ mock_launcher = MagicMock()
+ mock_launcher.config.get_default.return_value = MagicMock()
+ with patch("os.path.exists", return_value=True), \
+   patch("src.external_editor.get_default_launcher", return_value=mock_launcher), \
+   patch("src.external_editor.create_temp_modified_file", side_effect=RuntimeError("temp file creation blew up")):
+  result = gui_2._open_patch_in_external_editor_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._open_patch_in_external_editor_result"
+ assert "temp file creation blew up" in err.message