feat(bigframes): Add ai_generate functions to the dataframe bq accessor

[sycai]

internal issue: b/517233441

[gemini-code-assist]

Code Review

This pull request introduces AI generation accessor methods (generate, generate_bool, generate_int, and generate_double) to the AbstractBigQueryDataFrameAccessor class, along with corresponding unit tests. The feedback highlights a critical issue in several of the new unit tests where a mocked bpd.DataFrame is used, causing the tests to pass silently without actually executing the accessor code. It is recommended to use a real bpd.DataFrame fixture, such as scalar_types_df, to ensure the accessor implementations are properly exercised and verified.