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)

src/gui_2.py

@@ -1357,29 +1357,10 @@ class App:
 
  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
-   if not self._pending_patch_files:
-    self._patch_error_message = "No files to edit"
-    return
-   launcher = get_default_launcher(self.config)
-   editor   = launcher.config.get_default()
-   if not editor:
-    self._patch_error_message = "No external editor configured"
-    return
-   original_path = self._pending_patch_files[0]
-   if not os.path.exists(original_path):
-    self._patch_error_message = f"Original file not found: {original_path}"
-    return
-   temp_path = create_temp_modified_file(self._pending_patch_text)
-   result    = launcher.launch_diff(None, original_path, temp_path)
-   if result is None: self._patch_error_message = "Failed to launch external editor"
-   else:
-    self._patch_error_message = None
-    self._vscode_diff_process = result
-  except Exception as e:
-   self._patch_error_message = str(e)
+  ext_result = _open_patch_in_external_editor_result(self)
+  if not ext_result.ok:
+   if not hasattr(self, '_last_request_errors'): self._last_request_errors = []
+   self._last_request_errors.append(("_open_patch_in_external_editor", ext_result.errors[0]))
 
  def _reorder_ticket(self, src_idx: int, dst_idx: int) -> None:
   if src_idx == dst_idx: return
@@ -7818,6 +7799,70 @@ def _apply_pending_patch_result(app: "App") -> Result[bool]:
    original=e,
   )])
 
+
+def _open_patch_in_external_editor_result(app: "App") -> Result[bool]:
+ """Drain-aware variant of L1393 _open_patch_in_external_editor.
+
+ Extracts the external editor launch try/except from
+ App._open_patch_in_external_editor into a Result-returning helper. On
+ success (launcher.launch_diff returns a process), sets
+ app._patch_error_message=None, app._vscode_diff_process=result and returns
+ Result(data=True). On failure (launch returns None or raises), sets
+ app._patch_error_message and returns Result(data=False, errors=[ErrorInfo]).
+
+ The legacy wrapper drains errors to app._last_request_errors (per FR-BC-4
+ event-handler drain pattern; data plane attribute).
+
+ [C: src/gui_2.py:App._open_patch_in_external_editor (L1393 legacy wrapper)]
+ """
+ from src.external_editor import get_default_launcher, create_temp_modified_file
+ import os
+ try:
+  if not app._pending_patch_files:
+   app._patch_error_message = "No files to edit"
+   return Result(data=False, errors=[ErrorInfo(
+    kind=ErrorKind.INTERNAL,
+    message="No files to edit",
+    source="gui_2._open_patch_in_external_editor_result",
+   )])
+  launcher = get_default_launcher(app.config)
+  editor   = launcher.config.get_default()
+  if not editor:
+   app._patch_error_message = "No external editor configured"
+   return Result(data=False, errors=[ErrorInfo(
+    kind=ErrorKind.INTERNAL,
+    message="No external editor configured",
+    source="gui_2._open_patch_in_external_editor_result",
+   )])
+  original_path = app._pending_patch_files[0]
+  if not os.path.exists(original_path):
+   app._patch_error_message = f"Original file not found: {original_path}"
+   return Result(data=False, errors=[ErrorInfo(
+    kind=ErrorKind.INTERNAL,
+    message=f"Original file not found: {original_path}",
+    source="gui_2._open_patch_in_external_editor_result",
+   )])
+  temp_path = create_temp_modified_file(app._pending_patch_text)
+  result    = launcher.launch_diff(None, original_path, temp_path)
+  if result is None:
+   app._patch_error_message = "Failed to launch external editor"
+   return Result(data=False, errors=[ErrorInfo(
+    kind=ErrorKind.INTERNAL,
+    message="Failed to launch external editor",
+    source="gui_2._open_patch_in_external_editor_result",
+   )])
+  app._patch_error_message = None
+  app._vscode_diff_process = result
+  return Result(data=True)
+ except Exception as e:
+  app._patch_error_message = str(e)
+  return Result(data=False, errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"Open patch in external editor failed: {e}",
+   source="gui_2._open_patch_in_external_editor_result",
+   original=e,
+  )])
+
 #endregion: Phase 5 Event Handler Result Helpers
 
 #endregion: MMA