Fix regression in dataclass narrowing for Python >= 3.13 (#21675)

Fixes #21635

On Python >= 3.13, `@dataclass` synthesizes a `__replace__(self, ...) ->
Self`
method to support `copy.replace()`. When mypy tries to narrow a
`type[A]`
expression via `issubclass(cls, M)` (or `isinstance`) against a second,
unrelated dataclass `M`, it builds an ad-hoc `<subclass of A and M>`
type to
check whether that narrowing is sound (`intersect_instances` in
`checker.py`). Building that ad-hoc type runs
`check_multiple_inheritance`,
which sees `A`'s synthesized `__replace__` returning `A` and `M`'s
returning `M`, and flags them as incompatible.

That's a false positive: a real subclass of both (e.g. `class C(M, A)`,
itself decorated with `@dataclass`) gets its *own* freshly synthesized,
mutually compatible `__replace__`.