refactor: enhance type hints in diagnosis.py

[alingse]

Summary

• Replace Any with proper recursive JSONValue type definition
• Use cast() instead of type: ignore for type assertions
• Add Protocol classes for better type safety
• Improve function signatures with TypeVar
• Reduce type: ignore comments from 7 to 2

Details

Changes Made

1. JSONValue Type Definition

   • Changed from JSONValue = Any to a proper recursive type:

   JSONValue = Union[
       dict[str, "JSONValue"],
       list["JSONValue"],
       str, int, float, bool, None,
   ]

2. Protocol Classes Added

   • _ParseError: Define exception interface
   • _ExtendedDecoder: Define extended JSONDecoder interface
   • _JsonDecoderModule: Define json.decoder module interface

3. Type Assertions

   • Use cast(_ExtendedDecoder, ...) instead of # type: ignore[assignment]
   • Use cast(_ParseError, e) for exception handling

4. Function Signatures

   • _record_parser_name: Now uses TypeVar for better type inference

Test Plan

• mypy checks pass
• ruff checks pass
• All 108 pytest tests pass

Notes

The remaining 2 type: ignore comments are necessary for CPython internal
functions (py_scanstring, py_make_scanner) not exposed in typeshed stubs.