fix: Echo function_call.id into AFC function responses - #3201

$ python -I afc_id_probe.py   # the #3192 script, google/genai/__init__.py from a 634d88f export, google-genai 2.32.0, CPython 3.12
generate_content [{"id": "call_1", "name": "get_weather", ...}, {"id": "call_2", "name": "get_weather", ...}]
generate_content_stream [{"id": "call_1", ...}, {"id": "call_2", ...}]
# _extra_utils.py from the parent adbc79c: both lines lose their "id" again
$ python -m pytest -q google/genai/tests/afc
125 passed, 4 errors   # 634d88f; the 4 are replay tests that need credentials
120 passed, 4 errors   # adbc79c
# _extra_utils.py from adbc79c, the new tests kept: 4 failed, 6 passed