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()
+```

crates/ty_python_semantic/resources/mdtest/type_properties/is_assignable_to.md

@@ -672,6 +672,29 @@ def f(x: int, y: str) -> None: ...
 c1: Callable[[int], None] = partial(f, y="a")
 ```
 
+### Classes with `__call__` as attribute
+
+An instance type is assignable to a compatible callable type if the instance type's class has a
+callable `__call__` attribute.
+
+TODO: for the moment, we don't consider the callable type as a bound-method descriptor, but this may
+change for better compatibility with mypy/pyright.
+
+```py
+from typing import Callable
+from ty_extensions import static_assert, is_assignable_to
+
+def call_impl(a: int) -> str:
+    return ""
+
+class A:
+    __call__: Callable[[int], str] = call_impl
+
+static_assert(is_assignable_to(A, Callable[[int], str]))
+static_assert(not is_assignable_to(A, Callable[[int], int]))
+reveal_type(A()(1))  # revealed: str
+```
+
 ## Generics
 
 ### Assignability of generic types parameterized by gradual types

crates/ty_python_semantic/resources/mdtest/type_properties/is_subtype_of.md

@@ -1157,6 +1157,29 @@ def f(fn: Callable[[int], int]) -> None: ...
 f(a)
 ```
 
+### Classes with `__call__` as attribute
+
+An instance type can be a subtype of a compatible callable type if the instance type's class has a
+callable `__call__` attribute.
+
+TODO: for the moment, we don't consider the callable type as a bound-method descriptor, but this may
+change for better compatibility with mypy/pyright.
+
+```py
+from typing import Callable
+from ty_extensions import static_assert, is_subtype_of
+
+def call_impl(a: int) -> str:
+    return ""
+
+class A:
+    __call__: Callable[[int], str] = call_impl
+
+static_assert(is_subtype_of(A, Callable[[int], str]))
+static_assert(not is_subtype_of(A, Callable[[int], int]))
+reveal_type(A()(1))  # revealed: str
+```
+
 ### Class literals
 
 #### Classes with metaclasses