[red-knot] Synthesize a __call__ attribute for Callable types (astral-sh#17809)

## Summary

Currently this assertion fails on `main`, because we do not synthesize a
`__call__` attribute for Callable types:

```py
from typing import Protocol, Callable
from knot_extensions import static_assert, is_assignable_to

class Foo(Protocol):
    def __call__(self, x: int, /) -> str: ...

static_assert(is_assignable_to(Callable[[int], str], Foo))
```

This PR fixes that.

See previous discussion about this in
astral-sh#16493 (comment) and
astral-sh#17682 (comment)

## Test Plan

Existing mdtests updated; a couple of new ones added.

Author: AlexWaygood

crates/red_knot_python_semantic/resources/mdtest/annotations/callable.md

@@ -300,12 +300,7 @@ from typing import Callable
 def _(c: Callable[[int], int]):
     reveal_type(c.__init__)  # revealed: def __init__(self) -> None
     reveal_type(c.__class__)  # revealed: type
-
-    # TODO: The member lookup for `Callable` uses `object` which does not have a `__call__`
-    # attribute. We could special case `__call__` in this context. Refer to
-    # https://github.com/astral-sh/ruff/pull/16493#discussion_r1985098508 for more details.
-    # error: [unresolved-attribute] "Type `(int, /) -> int` has no attribute `__call__`"
-    reveal_type(c.__call__)  # revealed: Unknown
+    reveal_type(c.__call__)  # revealed: (int, /) -> int
 ```
 
 [gradual form]: https://typing.python.org/en/latest/spec/glossary.html#term-gradual-form

crates/red_knot_python_semantic/resources/mdtest/protocols.md

@@ -1476,26 +1476,32 @@ signature implied by the `Callable` type is assignable to the signature of the `
 specified by the protocol:
 
 ```py
+from knot_extensions import TypeOf
+
 class Foo(Protocol):
     def __call__(self, x: int, /) -> str: ...
 
-# TODO: these fail because we don't yet understand that all `Callable` types have a `__call__` method,
-# and we therefore don't think that the `Callable` type is assignable to `Foo`. They should pass.
-static_assert(is_subtype_of(Callable[[int], str], Foo))  # error: [static-assert-error]
-static_assert(is_assignable_to(Callable[[int], str], Foo))  # error: [static-assert-error]
+static_assert(is_subtype_of(Callable[[int], str], Foo))
+static_assert(is_assignable_to(Callable[[int], str], Foo))
 
-static_assert(not is_subtype_of(Callable[[str], str], Foo))
-static_assert(not is_assignable_to(Callable[[str], str], Foo))
-static_assert(not is_subtype_of(Callable[[CallMeMaybe, int], str], Foo))
-static_assert(not is_assignable_to(Callable[[CallMeMaybe, int], str], Foo))
+# TODO: these should pass
+static_assert(not is_subtype_of(Callable[[str], str], Foo))  # error: [static-assert-error]
+static_assert(not is_assignable_to(Callable[[str], str], Foo))  # error: [static-assert-error]
+static_assert(not is_subtype_of(Callable[[CallMeMaybe, int], str], Foo))  # error: [static-assert-error]
+static_assert(not is_assignable_to(Callable[[CallMeMaybe, int], str], Foo))  # error: [static-assert-error]
 
 def h(obj: Callable[[int], str], obj2: Foo, obj3: Callable[[str], str]):
-    # TODO: this fails because we don't yet understand that all `Callable` types have a `__call__` method,
-    # and we therefore don't think that the `Callable` type is assignable to `Foo`. It should pass.
-    obj2 = obj  # error: [invalid-assignment]
+    obj2 = obj
+
+    # TODO: we should emit [invalid-assignment] here because the signature of `obj3` is not assignable
+    # to the declared type of `obj2`
+    obj2 = obj3
+
+def satisfies_foo(x: int) -> str:
+    return "foo"
 
-    # This diagnostic is correct, however.
-    obj2 = obj3  # error: [invalid-assignment]
+static_assert(is_subtype_of(TypeOf[satisfies_foo], Foo))
+static_assert(is_assignable_to(TypeOf[satisfies_foo], Foo))
 ```
 
 ## Protocols are never singleton types, and are never single-valued types

crates/red_knot_python_semantic/src/types.rs

@@ -2953,6 +2953,10 @@ impl<'db> Type<'db> {
             Type::DataclassDecorator(_) => KnownClass::FunctionType
                 .to_instance(db)
                 .member_lookup_with_policy(db, name, policy),
+
+            Type::Callable(_) | Type::DataclassTransformer(_) if name_str == "__call__" => {
+                Symbol::bound(self).into()
+            }
             Type::Callable(_) | Type::DataclassTransformer(_) => KnownClass::Object
                 .to_instance(db)
                 .member_lookup_with_policy(db, name, policy),