[Python] Fix type annotations for tuple indexing, generic defaults, and reflection

[dbrattli]

Summary

• Fix tuple and union field indexing to use native Python int instead of Int32 wrapper
• Fix generic parameter defaults to use UNIT instead of None for type checker compatibility
• Update reflection functions to return Array instead of list for consistency
• Fix get_function_elements return type to tuple[TypeInfo, TypeInfo]
• Update indexed callback parameter types to use Int32 for contravariance compatibility
• Add or_else and or_else_with overloads for SomeWrapper[None] edge cases
• Simplify UNIT definition to UNIT: Any = None

This reduces the number of typing issues in the generated Python test code down to 1500 errors (down from 3000 when we started the typing improvements)