sorts: make comb_sort generic for comparable items

[otrisha]

Summary

. Make comb_sort generic over comparable item types
. Add doctest coverage for strings and floats
. Include comb_sort in the shared non-comparable-item rejection test

Part of #15234

AI assistance was used to help draft and review the implementation; I reviewed the changes before submission.

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[cclauss]

Awesome work! Thanks for doing this.