Fix Python schema conversion for variadic tuples with union elements

[林SO (Linxiushen)]

The Python schema converter raises AssertionError for variadic tuples whose elements are unions, such as tuple[int | str, ...], tuple[str | None, ...], or tuple[Literal["yes", "no"], ...]. This also prevents constructing a TypeChatJsonTranslator for a dataclass containing these fields.

Render these element types as Array<...>, matching the existing handling of lists with union elements. Extend the tuple schema snapshot with union, nullable, literal, and nested cases, and add a translator regression that validates a fixed JSON response into the expected Python tuples.

Validation on Python 3.12:

• The schema and translator regressions fail against the unchanged base; five existing translator tests pass.
• Full Python suite: 23 passed, including all 17 snapshots.
• Repository-pinned Pyright 1.1.358: 0 errors and 0 warnings across the Python project.
• git diff --check passes.

The translator test uses the existing fixed-response model; no live provider calls were made.