[ty] Add subtyping between SubclassOf and CallableType (#19026)

## Summary

Part of https://github.com/astral-sh/ty/issues/129

There were previously some false positives here.

## Test Plan

Updated `is_subtype_of.md` and `is_assignable_to.md`

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

@@ -1064,6 +1064,37 @@ static_assert(not is_assignable_to(A, Callable[[int], int]))
 reveal_type(A()(1))  # revealed: str
 ```
 
+### Subclass of
+
+#### Type of a class with constructor methods
+
+```py
+from typing import Callable
+from ty_extensions import static_assert, is_assignable_to
+
+class A:
+    def __init__(self, x: int) -> None: ...
+
+class B:
+    def __new__(cls, x: str) -> "B":
+        return super().__new__(cls)
+
+static_assert(is_assignable_to(type[A], Callable[[int], A]))
+static_assert(not is_assignable_to(type[A], Callable[[str], A]))
+
+static_assert(is_assignable_to(type[B], Callable[[str], B]))
+static_assert(not is_assignable_to(type[B], Callable[[int], B]))
+```
+
+#### Type with no generic parameters
+
+```py
+from typing import Callable, Any
+from ty_extensions import static_assert, is_assignable_to
+
+static_assert(is_assignable_to(type, Callable[..., Any]))
+```
+
 ## Generics
 
 ### Assignability of generic types parameterized by gradual types

crates/ty_python_semantic/resources/mdtest/type_properties/is_subtype_of.md

@@ -1752,6 +1752,28 @@ static_assert(not is_subtype_of(TypeOf[F], Callable[[], str]))
 static_assert(not is_subtype_of(TypeOf[F], Callable[[int], F]))
 ```
 
+### Subclass of
+
+#### Type of a class with constructor methods
+
+```py
+from typing import Callable
+from ty_extensions import TypeOf, static_assert, is_subtype_of
+
+class A:
+    def __init__(self, x: int) -> None: ...
+
+class B:
+    def __new__(cls, x: str) -> "B":
+        return super().__new__(cls)
+
+static_assert(is_subtype_of(type[A], Callable[[int], A]))
+static_assert(not is_subtype_of(type[A], Callable[[str], A]))
+
+static_assert(is_subtype_of(type[B], Callable[[str], B]))
+static_assert(not is_subtype_of(type[B], Callable[[int], B]))
+```
+
 ### Bound methods
 
 ```py

crates/ty_python_semantic/src/types.rs

@@ -1559,6 +1559,16 @@ impl<'db> Type<'db> {
                 .into_callable(db)
                 .has_relation_to(db, target, relation),
 
+            // TODO: This is unsound so in future we can consider an opt-in option to disable it.
+            (Type::SubclassOf(subclass_of_ty), Type::Callable(_))
+                if subclass_of_ty.subclass_of().into_class().is_some() =>
+            {
+                let class = subclass_of_ty.subclass_of().into_class().unwrap();
+                class
+                    .into_callable(db)
+                    .has_relation_to(db, target, relation)
+            }
+
             // `Literal[str]` is a subtype of `type` because the `str` class object is an instance of its metaclass `type`.
             // `Literal[abc.ABC]` is a subtype of `abc.ABCMeta` because the `abc.ABC` class object
             // is an instance of its metaclass `abc.ABCMeta`.