[red-knot] Correct modeling of dunder calls (#16368)

## Summary

Model dunder-calls correctly (and in one single place), by implementing
this behavior (using `__getitem__` as an example).

```py
def getitem_desugared(obj: object, key: object) -> object:
    getitem_callable = find_in_mro(type(obj), "__getitem__")
    if hasattr(getitem_callable, "__get__"):
        getitem_callable = getitem_callable.__get__(obj, type(obj))

    return getitem_callable(key)
```

See the new `calls/dunder.md` test suite for more information. The new
behavior also needs much fewer lines of code (the diff is positive due
to new tests).

## Test Plan

New tests; fix TODOs in existing tests.

crates/red_knot_python_semantic/resources/mdtest/binary/instances.md

@@ -259,11 +259,17 @@ class A:
 class B:
     __add__ = A()
 
-# TODO: this could be `int` if we declare `B.__add__` using a `Callable` type
-# TODO: Should not be an error: `A` instance is not a method descriptor, don't prepend `self` arg.
-#   Revealed type should be `Unknown | int`.
-# error: [unsupported-operator] "Operator `+` is unsupported between objects of type `B` and `B`"
-reveal_type(B() + B())  # revealed: Unknown
+reveal_type(B() + B())  # revealed: Unknown | int
+```
+
+Note that we union with `Unknown` here because `__add__` is not declared. We do infer just `int` if
+the callable is declared:
+
+```py
+class B2:
+    __add__: A = A()
+
+reveal_type(B2() + B2())  # revealed: int
 ```
 
 ## Integration test: numbers from typeshed