fix: log warnings on silently swallowed agent and endpoint failures (#2367)

get_builtin_overrides() was swallowing all exceptions with a bare
`except Exception: pass`, so misconfigured tool-description overrides
would silently produce wrong agent behaviour with no log trace.

The background endpoint refresh loop had the same pattern: any probe
failure was silently ignored, giving operators no signal that the
refresh was broken.

Also removes a circular self-import (`from src.agent_loop import
_build_base_prompt`) inside _build_system_prompt; the function is
already in scope and the import created a latent circular reference risk.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

routes/model_routes.py

@@ -993,8 +993,8 @@ def setup_model_routes(model_discovery):
                     db.close()
                 if changed:
                     _invalidate_models_cache()
-            except Exception:
-                pass
+            except Exception as e:
+                logger.warning('Background endpoint refresh failed: %s', e)
             finally:
                 for st in _refresh_state.values():
                     st["inflight"] = False