arg: fix order to use short form before long form (#18196)
* arg: fix order to use short form before long form
* arg: update doc
* arg: update test-arg-parser
* arg: address review feedback from ngxson
simplified to check first.length() <= last.length() only
fixed: --sampler-seq, --rerank, --draft ordering
note: middle positions in 3+ arg sets are not verified
* arg: update doc