[`flake8-builtins`] Expand checks in class scopes (`A001`) (#29076)
## Summary
Inside a class body, `flake8-builtins` reports a `for`, `with ... as`,
or `:=` target that shadows a builtin, but A001 does not. The A001 call
site skipped every stored name in class scope because A003 handles class
attributes, but A003 only considers assignment and annotation bindings,
so those other binding kinds were reported by neither rule. The
class-scope skip now applies only to the binding kinds A003 handles, by
checking the stored name's `BindingKind`. Attribute and method bindings
in a class still go to A003, so there's no double-reporting.
This is the gap left after #20178, which I picked up because no pull
request was opened for it.
Fixes #20179
## Test Plan
I have added new class-scope cases in the A001 fixture covering both the
three bindings that now fire and the attribute and method bindings that
stay silent, updated the snapshot with three new diagnostics, and ran
the `flake8-builtins` tests, `clippy`, and `fmt`.