Fix nthPermutation lexicographic order

[AaritPanda-git]

Fixes #10849

This PR fixes nthPermutation to always produce permutations in
lexicographic order, as implied by the Lehmer code representation
described in the documentation.

When the Lehmer code has leading zeros (idx < range.length),
the previous implementation lost these during reversal, resulting
in incorrect ordering. The fix preserves the leading zeros and
restores correct lexicographic behavior.

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Testing this PR locally

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[AaritPanda-git]

Gentle ping @PetarKirov in case this slipped through.
Happy to make any changes if needed.

[thewilsonator]

Apologies for missing this. Could you rebase to retrigger the CI.