add missing property decorator

debugging this was annoying

Author: matthewfranglen

pydantic_ai_slim/pydantic_ai/tools.py

@@ -568,6 +568,7 @@ def single_string_argument_name(self) -> str | None:
             return None
         return property_name
 
+    @property
     def defer(self) -> bool:
         """Whether calls to this tool will be deferred.