fix callback protocols

Author: AlexWaygood

crates/red_knot_python_semantic/resources/mdtest/protocols.md

@@ -1403,25 +1403,34 @@ static_assert(not is_fully_static(NoParameterAnnotation))  # error: [static-asse
 static_assert(not is_fully_static(NoReturnAnnotation))  # error: [static-assert-error]
 ```
 
-## `typing.SupportsIndex` and `typing.Sized`
+## Callable protocols
 
-`typing.SupportsIndex` is already somewhat supported through some special-casing in red-knot.
+An instance of a protocol type is callable if the protocol defines a `__call__` method:
 
 ```py
-from typing import SupportsIndex, Literal
+from typing import Protocol
 
-def _(some_int: int, some_literal_int: Literal[1], some_indexable: SupportsIndex):
-    a: SupportsIndex = some_int
-    b: SupportsIndex = some_literal_int
-    c: SupportsIndex = some_indexable
+class CallMeMaybe(Protocol):
+    def __call__(self, x: int) -> str: ...
+
+def _(obj: CallMeMaybe):
+    reveal_type(obj(42))  # revealed: str
+    obj("bar")  # error: [invalid-argument-type]
 ```
 
-The same goes for `typing.Sized`:
+## Integration test: `typing.SupportsIndex` and `typing.Sized`
+
+`typing.SupportsIndex` and `typing.Sized` are two protocols that are very commonly used in the wild.
 
 ```py
-from typing import Sized
+from typing import SupportsIndex, Sized, Literal
+
+def one(some_int: int, some_literal_int: Literal[1], some_indexable: SupportsIndex):
+    a: SupportsIndex = some_int
+    b: SupportsIndex = some_literal_int
+    c: SupportsIndex = some_indexable
 
-def _(some_list: list, some_tuple: tuple[int, str], some_sized: Sized):
+def two(some_list: list, some_tuple: tuple[int, str], some_sized: Sized):
     a: Sized = some_list
     b: Sized = some_tuple
     c: Sized = some_sized

crates/red_knot_python_semantic/src/types.rs

@@ -4060,7 +4060,7 @@ impl<'db> Type<'db> {
                 SubclassOfInner::Class(class) => Type::from(class).signatures(db),
             },
 
-            Type::NominalInstance(_) => {
+            Type::NominalInstance(_) | Type::ProtocolInstance(_) => {
                 // Note that for objects that have a (possibly not callable!) `__call__` attribute,
                 // we will get the signature of the `__call__` attribute, but will pass in the type
                 // of the original object as the "callable type". That ensures that we get errors