refactor(ai_client): narrow except in set_provider/set_tool_preset/set_bias_profile (Phase 9 sites 3+4+5)

Narrowed 3 INTERNAL_BROAD_CATCH sites to specific exception types:

1. set_provider (L394): except Exception -> except (OSError, ValueError)
   for the credential loading fallback

2. set_tool_preset (L520): except Exception -> except (OSError, ValueError, AttributeError)
   for tool preset loading (sys.stderr.write + flush preserved)

3. set_bias_profile (L537): except Exception -> except (OSError, ValueError, AttributeError)
   for bias profile loading (sys.stderr.write + flush preserved)

Sites 4-5 are now narrow+log patterns which the audit will classify as
INTERNAL_SILENT_SWALLOW (a violation per the styleguide's anti-sliming
rule). They will be addressed in Phase 11 (silent-swallow cleanup).

src/ai_client.py

@@ -391,7 +391,7 @@ def set_provider(provider: str, model: str, validate: bool = True) -> None:
   try:
    creds = _load_credentials()
    valid_models = _list_minimax_models(creds.get("minimax", {}).get("api_key", ""))
-  except Exception:
+  except (OSError, ValueError):
    valid_models = _list_minimax_models("")
   if model not in valid_models:
    _model = "MiniMax-M2.5"
@@ -517,7 +517,7 @@ def set_tool_preset(preset_name: Optional[str]) -> None:
       new_tools[name]            = True
       _tool_approval_modes[name] = tool.approval
     _agent_tools = new_tools
-  except Exception as e:
+  except (OSError, ValueError, AttributeError) as e:
    sys.stderr.write(f"[ERROR] Failed to set tool preset '{preset_name}': {e}\n")
    sys.stderr.flush()
  _CACHED_ANTHROPIC_TOOLS = None
@@ -534,7 +534,7 @@ def set_bias_profile(profile_name: Optional[str]) -> None:
    profiles = manager.load_all_bias_profiles()
    if profile_name in profiles:
     _active_bias_profile = profiles[profile_name]
-  except Exception as e:
+  except (OSError, ValueError, AttributeError) as e:
    sys.stderr.write(f"[ERROR] Failed to set bias profile '{profile_name}': {e}\n")
    sys.stderr.flush()