[ty] Use 'unknown' specialization for upper bound on Self (#20325)

## Summary

closes astral-sh/ty#1156

## Test Plan

Added a regression test

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/annotations/self.md

@@ -2,7 +2,7 @@
 
 ```toml
 [environment]
-python-version = "3.11"
+python-version = "3.13"
 ```
 
 `Self` is treated as if it were a `TypeVar` bound to the class it's being used on.
@@ -147,6 +147,23 @@ class Shape:
         return self
 ```
 
+## `Self` for classes with a default value for their generic parameter
+
+This is a regression test for <https://github.com/astral-sh/ty/issues/1156>.
+
+```py
+from typing import Self
+
+class Container[T = bytes]:
+    def __init__(self: Self, data: T | None = None) -> None:
+        self.data = data
+
+reveal_type(Container())  # revealed: Container[bytes]
+reveal_type(Container(1))  # revealed: Container[int]
+reveal_type(Container("a"))  # revealed: Container[str]
+reveal_type(Container(b"a"))  # revealed: Container[bytes]
+```
+
 ## Invalid Usage
 
 `Self` cannot be used in the signature of a function or variable.

crates/ty_python_semantic/src/types.rs

@@ -5711,9 +5711,7 @@ impl<'db> Type<'db> {
                             ],
                         });
                     };
-                    let instance = Type::ClassLiteral(class).to_instance(db).expect(
-                        "nearest_enclosing_class must return type that can be instantiated",
-                    );
+                    let instance = Type::instance(db, class.unknown_specialization(db));
                     let class_definition = class.definition(db);
                     let typevar = TypeVarInstance::new(
                         db,