Fix class access to functools.cached_property attributes

[aryansk]

Fixes #21825.

What

Accessing a functools.cached_property through the class object was typed as the raw getter callable, so descriptor attributes were reported as missing:

from functools import cached_property

class A:
    @cached_property
    def value(self) -> int:
        return 1

    @classmethod
    def name(cls) -> str:
        return cls.value.attrname  # error: "Callable[[A], int]" has no attribute "attrname"

At runtime A.value is the cached_property instance itself (it has attrname, func, etc.). This PR types class access as the descriptor and lets the existing descriptor machinery apply typeshed's __get__(self, instance: None, ...) -> Self overload:

reveal_type(A.value)  # functools.cached_property[builtins.int]

How

In analyze_class_attribute_access, when the member is decorated with functools.cached_property and accessed as a value (not an lvalue), feed the descriptor instance type (cached_property[<getter return>]) into analyze_descriptor_access instead of the bare callable. Instance access is unchanged (already went through the descriptor path), and lvalue behavior (e.g. A.value = 5) is unchanged.

Test

Added testCachedPropertyClassAccess to test-data/unit/check-functools.test; it fails on main and passes with this change. Full testcheck suite passes (8178 passed, 33 skipped, 7 xfailed).

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅