refactor(app_controller): migrate _fetch_models.do_fetch to per-provider Result (Phase 6 Group 6.4)

Replaces per-provider logging.debug body with _list_models_for_provider_result
SDK-boundary helper. Aggregates per-provider failures into self._model_fetch_errors
and returns Result with aggregated errors. Stderr summary on partial failure.

The SDK boundary (ai_client.list_models call) is the canonical place to
catch vendor exceptions and convert to ErrorInfo(kind=NETWORK), per
error_handling.md §'Boundary Types'.

Audit: INTERNAL_SILENT_SWALLOW for src/app_controller.py: 23 -> 22.

src/app_controller.py

@@ -845,6 +845,11 @@ class AppController:
   self._inject_preview_error:           Optional[ErrorInfo] = None
   self._mcp_config_parse_error:         Optional[ErrorInfo] = None
   self._save_project_error:             Optional[ErrorInfo] = None
+  # --- Per-provider model-fetch errors (Phase 6: Group 6.4) ---
+  # Aggregated per-provider failures from _fetch_models. The map is the
+  # durable data plane for sub-track 4 GUI to display in the models panel;
+  # stderr write IS the visible-but-incomplete drain (full drain = GUI).
+  self._model_fetch_errors:             Dict[str, ErrorInfo] = {}
   # --- Shared background pool + proactive warmup (startup_speedup_20260606) ---
   self._io_pool = make_io_pool()
   _install_sigint_exit_handler(self)
@@ -3322,6 +3327,27 @@ class AppController:
 
  #region: AI Settings
 
+ def _list_models_for_provider_result(self, p: str) -> "Result[List[str]]":
+  """SDK boundary (Phase 6 Group 6.4): wrap ai_client.list_models(p).
+  Returns Result[List[str]]. On failure: any SDK exception
+  (OSError/ValueError/TypeError/KeyError/AttributeError/RuntimeError/etc.)
+  -> ErrorInfo(kind=NETWORK, original=e).
+
+  Per the styleguide §"Boundary Types", SDK calls are the canonical
+  boundary: catch vendor exceptions and convert to ErrorInfo. The
+  per-provider accumulation in self._model_fetch_errors makes the
+  aggregated failures visible to sub-track 4 GUI."""
+  try:
+   models = ai_client.list_models(p)
+   return Result(data=list(models) if models else [])
+  except Exception as e:
+   return Result(data=[], errors=[ErrorInfo(
+    kind=ErrorKind.NETWORK,
+    message=str(e),
+    source=f"app_controller._list_models_for_provider_result[{p}]",
+    original=e,
+   )])
+
  def _fetch_models(self, provider: str) -> None:
   """
   [C: src/gui_2.py:App.run]
@@ -3336,12 +3362,16 @@ class AppController:
   self.ai_status = "fetching models..."
 
   def do_fetch() -> Result[None]:
+   aggregated_errors: List[ErrorInfo] = []
    try:
     for p in ai_client.PROVIDERS:
-     try:
-      self.all_available_models[p] = ai_client.list_models(p)
-     except (OSError, IOError, ValueError, TypeError, KeyError, AttributeError, RuntimeError) as e:
-      logging.getLogger(__name__).debug("list_models failed for %s: %s", p, e, extra={"source": "app_controller._fetch_models.do_fetch.per_provider"})
+     result = self._list_models_for_provider_result(p)
+     if result.ok:
+      self.all_available_models[p] = result.data
+     else:
+      err = result.errors[0]
+      self._model_fetch_errors[p] = err
+      aggregated_errors.append(err)
       self.all_available_models[p] = []
 
     models_list = self.all_available_models.get(provider, [])
@@ -3351,17 +3381,22 @@ class AppController:
      ai_client.set_provider(self._current_provider, self.current_model)
     if self.ai_status == "fetching models...":
      self.ai_status = f"models loaded: {len(models_list)}"
+    if aggregated_errors:
+     summary = f"{len(aggregated_errors)} provider(s) failed: " + "; ".join(e.message for e in aggregated_errors)
+     sys.stderr.write(f"model fetch partial failure: {summary}\n")
+     sys.stderr.flush()
+     return Result(data=None, errors=aggregated_errors)
     return OK
-   except (OSError, IOError, ValueError, TypeError, KeyError, AttributeError, RuntimeError) as e:
-    logging.getLogger(__name__).debug("model fetch error: %s", e, extra={"source": "app_controller._fetch_models.do_fetch"})
-    if self.ai_status == "fetching models...":
-     self.ai_status = f"model fetch error: {e}"
-    return Result(data=None, errors=[ErrorInfo(
+   except Exception as e:
+    err = ErrorInfo(
      kind=ErrorKind.INTERNAL,
      message=str(e),
      source="app_controller._fetch_models.do_fetch",
      original=e,
-    )])
+    )
+    if self.ai_status == "fetching models...":
+     self.ai_status = f"model fetch error: {e}"
+    return Result(data=None, errors=[err])
   self.submit_io(do_fetch)
 
  def _apply_preset(self, name: str, scope: str) -> None: