fix: Annotate response type as None for transfer_to_agent tool and set empty Schema as response schema when tool has no response annotation

1. if a function has no return type annotation, we should treat it as returning any type
2. we use empty schema (with `type` as None) to indicate no type constraints and this is already supported by model server

PiperOrigin-RevId: 789808104

tests/unittests/tools/test_from_function_with_options.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import Any
 from typing import Dict
 
 from google.adk.tools import _automatic_function_calling_util
@@ -51,9 +52,10 @@ def test_from_function_with_options_no_return_annotation_vertex():
   assert declaration.name == 'test_function'
   assert declaration.parameters.type == 'OBJECT'
   assert declaration.parameters.properties['param'].type == 'STRING'
-  # VERTEX_AI should have response schema for None return
+  # VERTEX_AI should have response schema for functions with no return annotation
+  # Changed: Now uses Any type instead of NULL for no return annotation
   assert declaration.response is not None
-  assert declaration.response.type == types.Type.NULL
+  assert declaration.response.type is None  # Any type maps to None in schema
 
 
 def test_from_function_with_options_explicit_none_return_vertex():
@@ -150,6 +152,26 @@ def test_from_function_with_options_int_return_vertex():
   assert declaration.response.type == types.Type.INTEGER
 
 
+def test_from_function_with_options_any_annotation_vertex():
+  """Test from_function_with_options with Any type annotation for VERTEX_AI."""
+
+  def test_function(param: Any) -> Any:
+    """A test function that uses Any type annotations."""
+    return param
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  # Any type should map to None in schema (TYPE_UNSPECIFIED behavior)
+  assert declaration.parameters.properties['param'].type is None
+  # VERTEX_AI should have response schema for Any return
+  assert declaration.response is not None
+  assert declaration.response.type is None  # Any type maps to None in schema
+
+
 def test_from_function_with_options_no_params():
   """Test from_function_with_options with no parameters."""