[red-knot] Check assignability of bound methods to callables (#17430)

## Summary

This is similar to https://github.com/astral-sh/ruff/pull/17095, it adds
assignability check for bound methods to callables.

## Test Plan

Add test cases to for assignability; specifically it uses gradual types
because otherwise it would just delegate to `is_subtype_of`.

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

@@ -504,4 +504,22 @@ c: Callable[[Any], str] = f
 c: Callable[[Any], str] = g
 ```
 
+### Method types
+
+```py
+from typing import Any, Callable
+
+class A:
+    def f(self, x: Any) -> str:
+        return ""
+
+    def g(self, x: Any) -> int:
+        return 1
+
+c: Callable[[Any], str] = A().f
+
+# error: [invalid-assignment] "Object of type `bound method A.g(x: Any) -> int` is not assignable to `(Any, /) -> str`"
+c: Callable[[Any], str] = A().g
+```
+
 [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

@@ -1368,6 +1368,10 @@ impl<'db> Type<'db> {
                     .is_assignable_to(db, target)
             }
 
+            (Type::BoundMethod(self_bound_method), Type::Callable(_)) => self_bound_method
+                .into_callable_type(db)
+                .is_assignable_to(db, target),
+
             // TODO other types containing gradual forms (e.g. generics containing Any/Unknown)
             _ => self.is_subtype_of(db, target),
         }