show: Preserve alias binders when printing open types (#61927)

Leverages the new invariant typevar semantics to fix a failure to
identify a type alias (missed case from the earlier
typevar identity work).

Noticed by GPT 5.5 in #61915, but is an unrelated bug.

---------

Co-authored-by: Keno Fischer <Keno@users.noreply.github.com>
Co-authored-by: OpenAI Codex <codex@openai.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>