Bind self for un-annotated Callable class attributes

[ANU-2524]

Class attributes whose type is a Callable (rather than a def) were only treated as methods when named like a dunder or annotated ClassVar[Callable]. Any other Callable stored in the class body was treated as a plain data attribute, so calling it on an instance did not drop self. This produced a false bad-argument-count for factory-produced methods such as PySpark's Column.isNull/asc/eqNullSafe, which are assigned as isNull = _unary_op(...) (issue #3465).

At runtime a plain function object stored on a class is a descriptor, so it binds self. mypy and pyright model this by binding self for any class-body Callable unless it carries an explicit, non-ClassVar annotation. is_method now follows the same rule: an inferred (un-annotated) Callable, a ClassVar[Callable], or a dunder binds self; an explicitly annotated bare Callable does not. This also makes a class-body lambda a bound method, matching mypy/pyright.

Summary

Fixes #3465

Test Plan

• Added regression tests covering factory-assigned Callable class attributes.
• Verified that instance access correctly binds self while class access does not.
• Confirmed behavior matches mypy and pyright for inferred Callable, explicit Callable, and ClassVar[Callable].
• Reproduced the original PySpark example (isNull, asc, desc, eqNullSafe) and verified the false bad-argument-count errors are resolved.