[huuan26]

Summary

This PR improves type annotations in simplejson.JSONDecoder:

1. Fix scan_once return type: previously typed as Callable[[str, int], tuple[bool, int]]. At runtime, scan_once parses any JSON value (string, number, array, object, bool, null) and returns tuple[Any, int] representing (parsed_value, end_idx).
2. Replace Incomplete with concrete types:
   • parse_string: returns tuple[str, int].
   • parse_object and parse_array: return tuple[Any, int].
3. Remove unused from _typeshed import Incomplete.
4. Add assert_type tests covering JSONDecoder methods in stubs/simplejson/@tests/test_cases/check_simplejson.py.

Verification

• flake8 stubs/simplejson: passed (0 errors)
• python tests/regr_test.py simplejson: passed with mypy
• python tests/stubtest_third_party.py simplejson: passed (success against installed simplejson)
• python tests/check_typeshed_structure.py: passed

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉