TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 5: refactor(gui_2): migrate L1428 request_patch_from_tier4 to Result[T] (Phase 5)

Extract request_patch_from_tier4_result helper from the
ai_client.run_tier4_patch_generation try/except in App.request_patch_from_tier4.
Legacy wrapper drains errors to app._last_request_errors per FR-BC-4
event-handler pattern.

[pre-audit] L1428 INTERNAL_BROAD_CATCH
[post-audit] V count: 10 -> 9 (L1428 removed)

src/gui_2.py

@@ -1382,20 +1382,10 @@ class App:
    self._push_mma_state_update()
 
  def request_patch_from_tier4(self, error: str, file_context: str) -> None:
-  try:
-   from src import ai_client
-   from src.diff_viewer import parse_diff
-   patch_text = ai_client.run_tier4_patch_generation(error, file_context)
-   if patch_text and "---" in patch_text and "+++" in patch_text:
-    diff_files                = parse_diff(patch_text)
-    file_paths                = [df.old_path for df in diff_files]
-    self._pending_patch_text  = patch_text
-    self._pending_patch_files = file_paths
-    self._show_patch_modal    = True
-   else:
-    self._patch_error_message = patch_text or "No patch generated"
-  except Exception as e:
-   self._patch_error_message = str(e)
+  tier4_result = request_patch_from_tier4_result(self, error, file_context)
+  if not tier4_result.ok:
+   if not hasattr(self, '_last_request_errors'): self._last_request_errors = []
+   self._last_request_errors.append(("request_patch_from_tier4", tier4_result.errors[0]))
 
  def bulk_execute(self) -> None:
   for tid in self.ui_selected_tickets:
@@ -7863,6 +7853,48 @@ def _open_patch_in_external_editor_result(app: "App") -> Result[bool]:
    original=e,
   )])
 
+
+def request_patch_from_tier4_result(app: "App", error: str, file_context: str) -> Result[bool]:
+ """Drain-aware variant of L1428 request_patch_from_tier4.
+
+ Extracts the ai_client.run_tier4_patch_generation try/except from
+ App.request_patch_from_tier4 into a Result-returning helper. On success
+ (patch_text has --- and +++), sets app._pending_patch_text/files and
+ app._show_patch_modal=True and returns Result(data=True). On invalid
+ patch (no --- or no +++) or exception, 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.request_patch_from_tier4 (L1428 legacy wrapper)]
+ """
+ from src.diff_viewer import parse_diff
+ try:
+  patch_text = ai_client.run_tier4_patch_generation(error, file_context)
+  if patch_text and "---" in patch_text and "+++" in patch_text:
+   diff_files                = parse_diff(patch_text)
+   file_paths                = [df.old_path for df in diff_files]
+   app._pending_patch_text  = patch_text
+   app._pending_patch_files = file_paths
+   app._show_patch_modal    = True
+   return Result(data=True)
+  else:
+   app._patch_error_message = patch_text or "No patch generated"
+   return Result(data=False, errors=[ErrorInfo(
+    kind=ErrorKind.INTERNAL,
+    message=f"Tier4 patch invalid or empty: {app._patch_error_message}",
+    source="gui_2.request_patch_from_tier4_result",
+   )])
+ except Exception as e:
+  app._patch_error_message = str(e)
+  return Result(data=False, errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"Tier4 patch generation failed: {e}",
+   source="gui_2.request_patch_from_tier4_result",
+   original=e,
+  )])
+
 #endregion: Phase 5 Event Handler Result Helpers
 
 #endregion: MMA