[ty] Fix CallableTypeOf[…] for classmethods (#20345)

## Summary

See https://github.com/astral-sh/ruff/pull/20338#discussion_r2337731998

## Test Plan

Regression test.

crates/ty_python_semantic/resources/mdtest/ty_extensions.md

@@ -481,6 +481,10 @@ class Foo:
     def returns_self(self, x: int) -> Self:
         return self
 
+    @classmethod
+    def class_method(cls, x: int) -> Self:
+        return cls(x)
+
 def _(
     c1: CallableTypeOf[f1],
     c2: CallableTypeOf[f2],
@@ -488,6 +492,7 @@ def _(
     c4: CallableTypeOf[Foo],
     c5: CallableTypeOf[Foo(42).__call__],
     c6: CallableTypeOf[Foo(42).returns_self],
+    c7: CallableTypeOf[Foo.class_method],
 ) -> None:
     reveal_type(c1)  # revealed: () -> Unknown
     reveal_type(c2)  # revealed: () -> int
@@ -499,4 +504,5 @@ def _(
     reveal_type(c5)  #  revealed: (x: int) -> str
 
     reveal_type(c6)  # revealed: (x: int) -> Foo
+    reveal_type(c7)  # revealed: (x: int) -> Foo
 ```

crates/ty_python_semantic/src/types/infer/builder/type_expression.rs

@@ -1166,7 +1166,7 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     if let Some(bound_method) = argument_type.into_bound_method() {
                         binding
                             .signature
-                            .bind_self(self.db(), Some(bound_method.self_instance(db)))
+                            .bind_self(self.db(), Some(bound_method.typing_self_type(db)))
                     } else {
                         binding.signature.clone()
                     }