fix(openapi): tighten /run handler error path + drop cargo-cult call

- Drop except ValueError: raise. Inherited from per-mode controllers
  without examining purpose; today it converts helper-internal
  ValueErrors into uncaptured 500s with no body or log. Falling
  through to except Exception: gives them a logged trace and a
  structured InternalServerError.
- Drop redundant AppMode.value_of(app_model.mode). App.mode is
  Mapped[AppMode] with an EnumText adapter that returns the enum
  directly; value_of was a no-op iteration.
- Comment the explicit re-raise block to spell out why ordering
  matters before the catch-all.

api/controllers/openapi/app_run.py

@@ -220,20 +220,19 @@ class AppRunApi(Resource):
         except ValidationError as exc:
             raise UnprocessableEntity(exc.json())
 
-        mode = AppMode.value_of(app_model.mode)
+        mode = app_model.mode
         handler = _DISPATCH.get(mode)
         if handler is None:
             raise UnprocessableEntity("mode_not_runnable")
 
         streaming = payload.response_mode == "streaming"
+        # Preserve specific HTTPException codes that the catch-all would otherwise mask.
         try:
             stream_obj, blocking_body = handler(app_model, caller, payload, streaming)
         except UnprocessableEntity:
             raise
         except (NotChatAppError, NotWorkflowAppError):
             raise
-        except ValueError:
-            raise
         except Exception:
             logger.exception("internal server error.")
             raise InternalServerError()