[red-knot] Assignability of class instances to Callable (#17590)
## Summary
Model assignability of class instances with a `__call__` method to
`Callable` types. This should solve some false positives related to
`functools.partial` (yes, 1098 fewer diagnostics!).
Reference:
https://github.com/astral-sh/ruff/issues/17343#issuecomment-2824618483
## Test Plan
New Markdown tests.
