[copybara-service]

fix: Drop the whole AFC exchange from curated chat history when the final model turn is invalid

fixes #3314

FUTURE_COPYBARA_INTEGRATE_REVIEW=#3249 from googleapis:release-please--branches--main 4b4b0f3

[chrikrah]

The live AFC half fixes #3314. The rebuild half, which I asked for there, splits from the live path.

# git archive 1ffa74f (base afb10e8), uv venv, Python 3.12, genai.__file__ printed per tree
$ python -I afc_tail.py    # the script from #3314
afb10e8  curated: ['user:text', 'model:function_call']
1ffa74f  curated: []

$ python -m pytest google/genai/tests/chats/test_get_history.py -q
27 passed in 0.11s                    # 1ffa74f
7 failed, 20 passed in 0.29s          # chats.py from afb10e8, tests kept
# not run: the replay tests in tests/chats, which need credentials

non-blocking: a stored history can't tell AFC from manual calls. Send a function_response by hand, get a blocked answer, and only the rebuild drops the call:

$ python -I manual_fc.py    # FunctionDeclaration tool, send_message(Part.from_function_response(...)), SAFETY reply
afb10e8  live: ['user:text', 'model:function_call']  rebuilt via chats.create(history=...): same
1ffa74f  live: ['user:text', 'model:function_call']  rebuilt via chats.create(history=...): []

The caller can resend that response, and 2580638 ends turns on a call on purpose. My pick: drop _drop_rejected_input and its hand-built test and accept the #3314 tail on a restored AFC history. Or should the live manual path drop the call too?