chore(bigframes): refactor "generate_bigframes_bigquery.py"

[sycai]

Changes:

• Separated file parsing logic from code generation, and modularized the code
• Replaced nested dictionary with data classes for better modeling of data
• Let the old generate_bigframes_bigquery.py call the new Python module

The old code generation CLI still works.

I generated all the code files and checked the results. The only thing changed in the output are script paths in the file comment, which is intended.

[gemini-code-assist]

Code Review

This pull request refactors the BigQuery code generator by modularizing the monolithic generate_bigframes_bigquery.py script into a structured package under bigquery_generator/. The feedback is highly constructive and focuses on improving the refactored code. Key recommendations include fixing the relative script path calculation to preserve descriptive headers in generated files, using relative imports for sibling modules within the new package, explicitly specifying UTF-8 encoding when opening files, adding or correcting type hints (including a type mismatch in render_accessor), and adjusting the sys.path modification to import the generator as a proper package.