[ty] Fix CallableTypeOf[…] for bound methods (#20338)

## Summary

`CallableTypeOf[bound_method]` would previously bind `self` to the
bound method type itself, instead of binding it to the instance type
stored inside the bound method type.

## Test Plan

Added regression test

crates/ty_python_semantic/src/types.rs

@@ -1046,8 +1046,11 @@ impl<'db> Type<'db> {
         matches!(self, Type::FunctionLiteral(..))
     }
 
-    pub(crate) const fn is_bound_method(&self) -> bool {
-        matches!(self, Type::BoundMethod(..))
+    pub(crate) const fn into_bound_method(self) -> Option<BoundMethodType<'db>> {
+        match self {
+            Type::BoundMethod(bound_method) => Some(bound_method),
+            _ => None,
+        }
     }
 
     pub(crate) fn is_union_of_single_valued(&self, db: &'db dyn Db) -> bool {

crates/ty_python_semantic/src/types/infer.rs

@@ -11040,8 +11040,10 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     .expect("`Bindings` should have at least one `CallableBinding`");
 
                 let mut signature_iter = callable_binding.into_iter().map(|binding| {
-                    if argument_type.is_bound_method() {
-                        binding.signature.bind_self(self.db(), Some(argument_type))
+                    if let Some(bound_method) = argument_type.into_bound_method() {
+                        binding
+                            .signature
+                            .bind_self(self.db(), Some(bound_method.self_instance(db)))
                     } else {
                         binding.signature.clone()
                     }