[ty] Repurpose FunctionType.into_bound_method_type to return BoundMethodType (#19793)

## Summary

As per our naming scheme (at least for callable types) this should
return a `BoundMethodType`, or be renamed, but it makes more sense to
change the return type.

I also ensure `ClassType.into_callable` returns a `Type::Callable` in
the changed branch.

Ideally we could return a `CallableType` from these `into_callable`
functions (and rename to `into_callable_type` but because of unions we
cannot do this.

crates/ty_python_semantic/src/types/class.rs

@@ -1044,7 +1044,11 @@ impl<'db> ClassType<'db> {
                     .place;
 
                 if let Place::Type(Type::FunctionLiteral(new_function), _) = new_function_symbol {
-                    new_function.into_bound_method_type(db, self_ty)
+                    Type::Callable(
+                        new_function
+                            .into_bound_method_type(db, self_ty)
+                            .into_callable_type(db),
+                    )
                 } else {
                     // Fallback if no `object.__new__` is found.
                     CallableType::single(

crates/ty_python_semantic/src/types/function.rs

@@ -800,13 +800,13 @@ impl<'db> FunctionType<'db> {
         CallableType::new(db, self.signature(db), false)
     }
 
-    /// Convert the `FunctionType` into a [`Type::BoundMethod`].
+    /// Convert the `FunctionType` into a [`BoundMethodType`].
     pub(crate) fn into_bound_method_type(
         self,
         db: &'db dyn Db,
         self_instance: Type<'db>,
-    ) -> Type<'db> {
-        Type::BoundMethod(BoundMethodType::new(db, self, self_instance))
+    ) -> BoundMethodType<'db> {
+        BoundMethodType::new(db, self, self_instance)
     }
 
     pub(crate) fn has_relation_to(