Fix codegen with from keyword

[daweifeng-replit]

Why

For schema with from and to properties. from is a Python reserved keyword, and the river-python codegen was generating invalid Python like:

class Rewrites(TypedDict):
    from: NotRequired[str | None]   # SyntaxError!

What changed

• src/replit_river/codegen/typing.py — Added import keyword and extended normalize_special_chars to append _ to Python keywords (e.g., from -> from_). The existing alias logic in client.py already handles setting Field(alias="from") for BaseModel when the field name is normalized, so no changes needed there.

• tests/v1/codegen/test_input_special_chars.py — Added two new tests (test_python_keyword_field_names_basemodel and test_python_keyword_field_names_typeddict) that verify the codegen produces valid Python when schema fields use reserved keywords like from, class, and import.

Test plan

Added new tests