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e19c0503 - [ty] disallow explicit specialization of type variables themselves (#21938)

[ty] disallow explicit specialization of type variables themselves (#21938) ## Summary This PR makes explicit specialization of a type variable itself an error, and the result of the specialization is `Unknown`. The change also fixes https://github.com/astral-sh/ty/issues/1794. ## Test Plan mdtests updated new corpus test --------- Co-authored-by: Carl Meyer <carl@astral.sh>