A001 misses some attributes referenced within the class scope

### Summary

[`builtin-variable-shadowing` (A001)](https://docs.astral.sh/ruff/rules/builtin-variable-shadowing/) ignores some bindings in class scope which it checks in other scopes. [Example](https://play.ruff.rs/1ab5858c-b390-4f6a-9f89-0b98ef1cf5d9):
```console
$ cat >a001.py <<'# EOF'
class C:
for id in [1]: pass
def f1(self, x=id): return x

(id := 2)
def f2(self, x=id): return x

from contextlib import nullcontext
with nullcontext() as id: pass
def f3(self, x=id): return x

c = C()
print(c.f1(), c.f2(), c.f3())
# EOF

$ python a001.py
1 2 None

$ ruff --isolated check a001.py --select A001
All checks passed!

$ flake8 --select A a001.py
a001.py:2:5: A001 variable "id" is shadowing a Python builtin
a001.py:5:6: A001 variable "id" is shadowing a Python builtin
a001.py:9:5: A001 variable "id" is shadowing a Python builtin
```

It might make more sense to report these for [`builtin-attribute-shadowing` (A003)](https://docs.astral.sh/ruff/rules/builtin-attribute-shadowing/), but A001 is how the upstream plugin categorizes them. Either way, they are false negatives.

### Version

ruff 0.12.11 (c2bc15bc1 2025-08-28)

Use the a001.py reproducer and run Ruff with --isolated check --select A001, comparing its output with flake8. Trace the A001 and A003 rule entry points to determine which class-scope bindings are covered, then add regression coverage so the reported cases are no longer false negatives.

Written by the indexing model from the issue text.