fix: stop masking runtime failures

tests/config/test_model_presets.py

@@ -16,6 +16,11 @@ def test_resolve_preset_returns_defaults_when_no_preset() -> None:
     assert resolved.reasoning_effort == config.agents.defaults.reasoning_effort
 
 
+def test_agent_timezone_rejects_unknown_iana_name() -> None:
+    with pytest.raises(ValueError, match="unknown timezone"):
+        Config.model_validate({"agents": {"defaults": {"timezone": "Not/AZone"}}})
+
+
 def test_provider_api_type_accepts_exact_values_only() -> None:
     config = Config.model_validate({
         "providers": {