[ty] Handle typevars that have other typevars as a default (#17956)

It's possible for a typevar to list another typevar as its default
value:

```py
class C[T, U = T]: ...
```

When specializing this class, if a type isn't provided for `U`, we would
previously use the default as-is, leaving an unspecialized `T` typevar
in the specialization. Instead, we want to use what `T` is mapped to as
the type of `U`.

```py
reveal_type(C())  # revealed: C[Unknown, Unknown]
reveal_type(C[int]())  # revealed: C[int, int]
reveal_type(C[int, str]())  # revealed: C[int, str]
```

This is especially important for the `slice` built-in type.

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -1945,7 +1945,7 @@ reveal_type(C.a_complex)  # revealed: int | float | complex
 reveal_type(C.a_tuple)  # revealed: tuple[int]
 reveal_type(C.a_range)  # revealed: range
 # TODO: revealed: slice[Any, Literal[1], Any]
-reveal_type(C.a_slice)  # revealed: slice[Any, _StartT_co, _StartT_co | _StopT_co]
+reveal_type(C.a_slice)  # revealed: slice[Any, Any, Any]
 reveal_type(C.a_type)  # revealed: type
 reveal_type(C.a_none)  # revealed: None
 ```

crates/ty_python_semantic/resources/mdtest/generics/legacy/variables.md

@@ -86,6 +86,26 @@ S = TypeVar("S")
 reveal_type(S.__default__)  # revealed: NoDefault
 ```
 
+### Using other typevars as a default
+
+```py
+from typing import Generic, TypeVar, Union
+
+T = TypeVar("T")
+U = TypeVar("U", default=T)
+V = TypeVar("V", default=Union[T, U])
+
+class Valid(Generic[T, U, V]): ...
+
+reveal_type(Valid())  # revealed: Valid[Unknown, Unknown, Unknown]
+reveal_type(Valid[int]())  # revealed: Valid[int, int, int]
+reveal_type(Valid[int, str]())  # revealed: Valid[int, str, int | str]
+reveal_type(Valid[int, str, None]())  # revealed: Valid[int, str, None]
+
+# TODO: error, default value for U isn't available in the generic context
+class Invalid(Generic[U]): ...
+```
+
 ### Type variables with an upper bound
 
 ```py

crates/ty_python_semantic/resources/mdtest/generics/pep695/variables.md

@@ -40,6 +40,25 @@ def g[S]():
     reveal_type(S.__default__)  # revealed: NoDefault
 ```
 
+### Using other typevars as a default
+
+```toml
+[environment]
+python-version = "3.13"
+```
+
+```py
+class Valid[T, U = T, V = T | U]: ...
+
+reveal_type(Valid())  # revealed: Valid[Unknown, Unknown, Unknown]
+reveal_type(Valid[int]())  # revealed: Valid[int, int, int]
+reveal_type(Valid[int, str]())  # revealed: Valid[int, str, int | str]
+reveal_type(Valid[int, str, None]())  # revealed: Valid[int, str, None]
+
+# error: [unresolved-reference]
+class Invalid[S = T]: ...
+```
+
 ### Type variables with an upper bound
 
 ```py