* fix: skip FunctionCallingConfig when only native tools are present
When native tools (google_search, url_context) are enabled without any
function_declarations, _prepare_query_config was still creating a
FunctionCallingConfig, which makes Gemini API return 400 INVALID_ARGUMENT.
Now we only set tool_config when tool_list actually contains
function_declarations.
Fixes#7406
* style: ruff format