fix: warn about AFC with forced function calls

[uuzzrm]

Summary

• warn when FunctionCallingConfig.mode=ANY is used while automatic function calling is enabled
• cover enabled, disabled, AUTO, and default AFC configuration paths

Why

ANY forces every model turn to produce a function call. When automatic function calling is also enabled, the SDK can consume the configured remote-call budget without producing a natural-language response. This remains a valid configuration for callers managing the loop themselves, so the SDK reports the interaction with a warning rather than rejecting the request.

Testing

• uv run --no-project --with-requirements requirements.txt python -m pytest google/genai/tests/afc/test_raise_error_for_afc_incompatible_config.py -q
• uv run --no-project --with-requirements requirements.txt python -m pytest google/genai/tests/afc -q --ignore=google/genai/tests/afc/test_generate_content_stream_afc_thoughts.py
• python -m compileall on the changed modules
• git diff --check

The full AFC directory also reached 116 passing tests; four existing replay-based streaming cases require Google credentials that are not available in this environment.

Fixes #2839