[ty] Add "Did you mean" suggestion for Callable[[...], R]
When `...` is the sole element inside the parameter list of a
`Callable` type (i.e. `Callable[[...], int]`), add a subdiagnostic
suggesting the correct gradual form: `Callable[..., <return type>]`.
This is the only ellipsis-in-type-expression case where the user's
intent is unambiguous enough to warrant a suggestion. Other cases
(e.g. `Callable[[int, ...], R]`, `Union[int, ...]`, `list[...]`)
are left without suggestions since the intended type is unclear.
https://claude.ai/code/session_01KDdJ3EZbcBiVeEADaLjPdy
