`UP040` fix produces invalid syntax when a defaulted TypeVar precedes a non-defaulted TypeVar

[ntBre]

Summary

The preview UP040 fix can emit an invalid PEP 695 type-parameter order when a defaulted TypeVar is followed by a non-defaulted TypeVar.

Playground reproduction

from typing import TypeAlias, TypeVar

T = TypeVar("T", default=int)
S = TypeVar("S")

Pair: TypeAlias = tuple[T, S]

With --target-version py313 --preview --select UP040 --unsafe-fixes, Ruff produces:

type Pair[T = int, S] = tuple[T, S]

Ruff then rejects its own output:

invalid-syntax: non default type parameter `S` follows default type parameter

UP040 should not offer a fix in this case unless it can generate a valid and equivalent type-parameter list. This blocks #26948.

This also affects UP046 and UP047.