fix: Exclude additionalProperties from Gemini schemas

PiperOrigin-RevId: 820542466

Author: GWeale

src/google/adk/tools/_gemini_schema_util.py

@@ -142,7 +142,9 @@ def _sanitize_schema_formats_for_gemini(
 ) -> dict[str, Any]:
  """Filters the schema to only include fields that are supported by JSONSchema."""
  supported_fields: set[str] = set(_ExtendedJSONSchema.model_fields.keys())
- schema_field_names: set[str] = {"items"} # 'additional_properties' to come
+ # Gemini rejects schemas that include `additionalProperties`, so drop it.
+ supported_fields.discard("additional_properties")
+ schema_field_names: set[str] = {"items"}
  list_schema_field_names: set[str] = {
  "any_of", # 'one_of', 'all_of', 'not' to come
  }