refactor: remove redundant InitInputType class attribute

Add _get_init_input_type() classmethod that infers the init input type
from the generic parameter Function[T] using __orig_bases__ and
get_args(). This eliminates the need for explicit InitInputType class
attributes in subclasses.

- Add _get_init_input_type() with per-class caching
- Remove InitInputType from ScalarFunctionGenerator
- Remove InitInputType from TableFunctionBase

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rustyconover