[red-knot] Assignability of class instances to Callable (#17590)

## Summary

Model assignability of class instances with a `__call__` method to
`Callable` types. This should solve some false positives related to
`functools.partial` (yes, 1098 fewer diagnostics!).

Reference:
https://github.com/astral-sh/ruff/issues/17343#issuecomment-2824618483

## Test Plan

New Markdown tests.

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

@@ -560,4 +560,30 @@ c: Callable[..., int] = overloaded
 c: Callable[[int], str] = overloaded
 ```
 
+### Classes with `__call__`
+
+```py
+from typing import Callable, Any
+from knot_extensions import static_assert, is_assignable_to
+
+class TakesAny:
+    def __call__(self, a: Any) -> str:
+        return ""
+
+class ReturnsAny:
+    def __call__(self, a: str) -> Any: ...
+
+static_assert(is_assignable_to(TakesAny, Callable[[int], str]))
+static_assert(not is_assignable_to(TakesAny, Callable[[int], int]))
+
+static_assert(is_assignable_to(ReturnsAny, Callable[[str], int]))
+static_assert(not is_assignable_to(ReturnsAny, Callable[[int], int]))
+
+from functools import partial
+
+def f(x: int, y: str) -> None: ...
+
+c1: Callable[[int], None] = partial(f, y="a")
+```
+
 [typing documentation]: https://typing.python.org/en/latest/spec/concepts.html#the-assignable-to-or-consistent-subtyping-relation

crates/red_knot_python_semantic/src/types.rs

@@ -1460,6 +1460,16 @@ impl<'db> Type<'db> {
                 self_callable.is_assignable_to(db, target_callable)
             }
 
+            (Type::Instance(_), Type::Callable(_)) => {
+                let call_symbol = self.member(db, "__call__").symbol;
+                match call_symbol {
+                    Symbol::Type(Type::BoundMethod(call_function), _) => call_function
+                        .into_callable_type(db)
+                        .is_assignable_to(db, target),
+                    _ => false,
+                }
+            }
+
             (Type::FunctionLiteral(self_function_literal), Type::Callable(_)) => {
                 self_function_literal
                     .into_callable_type(db)