refactor(bigframes): reduce code size of AI functions and tests

[sycai]

The changes include:

• Make all optional parameters default to None, which reduces the length of compiled SQLs
• "Upper" the parameters at the API layer if necessary, so that we don't need to repeat this logic in both compilers later
• Make all optional parameters default to None in the operator definitions too, saving some lines in the unit tests

[gemini-code-assist]

Code Review

This pull request refactors AI operations to handle optional parameters more consistently by defaulting them to None and moving string normalization to an earlier stage using a new _upper_optional helper. It also updates type hints and literal values to uppercase to align with BigQuery expectations. The review feedback suggests avoiding shadowing built-in functions in the new helper and recommends expanding the use of Literal type hints for task_type and optimization_mode to further improve type safety and consistency across the codebase.