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

src/agent_loop.py

@@ -375,7 +375,8 @@ def get_builtin_overrides() -> dict:
         from src.settings import get_setting
         ov = get_setting("builtin_tool_overrides", {})
         return ov if isinstance(ov, dict) else {}
-    except Exception:
+    except Exception as e:
+        logger.warning('Failed to load builtin tool overrides: %s', e)
         return {}
 
 
@@ -611,8 +612,7 @@ def _build_system_prompt(
         # Skill index is user-editable (name + description), so it must never
         # live in the trusted system role and is NOT cached. Always recompute
         # when the cache hits.
-        from src.agent_loop import _build_base_prompt as _bbp_recompute
-        _, _skill_index_block = _bbp_recompute(
+        _, _skill_index_block = _build_base_prompt(
             disabled_tools, mcp_mgr, needs_admin, relevant_tools,
             mcp_disabled_map=mcp_disabled_map, compact=compact,
         )