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

Author: GWeale

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
 

tests/unittests/models/test_litellm.py

@@ -1066,6 +1066,40 @@ def test_function_declaration_to_tool_param(
   )
 
 
+def test_function_declaration_to_tool_param_without_required_attribute():
+  """Ensure tools without a required field attribute don't raise errors."""
+
+  class SchemaWithoutRequired:
+    """Mimics a Schema object that lacks the required attribute."""
+
+    def __init__(self):
+      self.properties = {
+          "optional_arg": types.Schema(type=types.Type.STRING),
+      }
+
+  func_decl = types.FunctionDeclaration(
+      name="function_without_required_attr",
+      description="Function missing required attribute",
+  )
+  func_decl.parameters = SchemaWithoutRequired()
+
+  expected = {
+      "type": "function",
+      "function": {
+          "name": "function_without_required_attr",
+          "description": "Function missing required attribute",
+          "parameters": {
+              "type": "object",
+              "properties": {
+                  "optional_arg": {"type": "string"},
+              },
+          },
+      },
+  }
+
+  assert _function_declaration_to_tool_param(func_decl) == expected
+
+
 def test_function_declaration_to_tool_param_with_parameters_json_schema():
   """Ensure function declarations using parameters_json_schema are handled.