ANN2 fixes can add return types that are shadowed

[dscorbett]

Summary

The fixes for missing-return-type-undocumented-public-function (ANN201), missing-return-type-private-function (ANN202), missing-return-type-special-method (ANN204), missing-return-type-static-method (ANN205), and missing-return-type-class-method (ANN206) add return types without checking for shadowed variables, which can cause type-checking errors. They should use get_or_import_builtin_symbol or similar. Example:

$ cat >ann2.py <<'# EOF'
from collections import UserString as str
from typing import override
def foo(): return "!"
def _foo(): return "!"
class C:
    @override
    def __str__(self): return "!"
    @staticmethod
    def foo(): return "!"
    @classmethod
    def bar(cls): return "!"
# EOF

$ ruff --isolated check --select ANN201,ANN202,ANN204,ANN205,ANN206 ann2.py --unsafe-fixes --fix
Found 5 errors (5 fixed, 0 remaining).

$ mypy ann2.py
ann2.py:3: error: Incompatible return value type (got "str", expected "UserString")  [return-value]
ann2.py:4: error: Incompatible return value type (got "str", expected "UserString")  [return-value]
ann2.py:7: error: Return type "UserString" of "__str__" incompatible with return type "str" in supertype "builtins.object"  [override]
ann2.py:7: error: Incompatible return value type (got "str", expected "UserString")  [return-value]
ann2.py:9: error: Incompatible return value type (got "str", expected "UserString")  [return-value]
ann2.py:11: error: Incompatible return value type (got "str", expected "UserString")  [return-value]
Found 6 errors in 1 file (checked 1 source file)

Version

ruff 0.13.2 (b0bdf03 2025-09-25)