Fix instance vs callable subtyping/assignability (#18260)

## Summary

Fix some issues with subtying/assignability for instances vs callables.
We need to look up dunders on the class, not the instance, and we should
limit our logic here to delegating to the type of `__call__`, so it
doesn't get out of sync with the calls we allow.

Also, we were just entirely missing assignability handling for
`__call__` implemented as anything other than a normal bound method
(though we had it for subtyping.)

A first step towards considering what else we want to change in
https://github.com/astral-sh/ty/issues/491

## Test Plan

mdtests

---------

Co-authored-by: med <medioqrity@gmail.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ty_python_semantic/resources/mdtest/call/dunder.md

@@ -239,3 +239,37 @@ def _(flag: bool):
     # error: [possibly-unbound-implicit-call]
     reveal_type(c[0])  # revealed: str
 ```
+
+## Dunder methods cannot be looked up on instances
+
+Class-level annotations with no value assigned are considered instance-only, and aren't available as
+dunder methods:
+
+```py
+from typing import Callable
+
+class C:
+    __call__: Callable[..., None]
+
+# error: [call-non-callable]
+C()()
+
+# error: [invalid-assignment]
+_: Callable[..., None] = C()
+```
+
+And of course the same is true if we have only an implicit assignment inside a method:
+
+```py
+from typing import Callable
+
+class C:
+    def __init__(self):
+        self.__call__ = lambda *a, **kw: None
+
+# error: [call-non-callable]
+C()()
+
+# error: [invalid-assignment]
+_: Callable[..., None] = C()
+```