fix: Safely handle FunctionDeclaration without a required attribute

Update `_function_declaration_to_tool_param` to use `getattr` when accessing the `required` field within `function_declaration.parameters`. This prevents `AttributeError` when a `FunctionDeclaration`'s parameters schema does not include a `required` attribute
Close #2891

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 830225582

src/google/adk/models/lite_llm.py

@@ -525,13 +525,13 @@ def _function_declaration_to_tool_param(
       },
   }
 
-  if (
-      function_declaration.parameters
-      and function_declaration.parameters.required
-  ):
-    tool_params["function"]["parameters"][
-        "required"
-    ] = function_declaration.parameters.required
+  required_fields = (
+      getattr(function_declaration.parameters, "required", None)
+      if function_declaration.parameters
+      else None
+  )
+  if required_fields:
+    tool_params["function"]["parameters"]["required"] = required_fields
 
   return tool_params