refs #14616 - use `std::unordered_set` in `TokenList::validateAst()` #19573

Job	Run time
build_cmake (ubuntu:24.04, true, true)	2m 35s
build_make (ubuntu:25.10)	3m 23s
build_make (alpine:3.23)	5m 47s
build_make (ubuntu:24.04)	4m 22s
build_cmake (alpine:3.23, false, false)	4m 16s
build_cmake (ubuntu:25.10, true, true)	5m 51s
	26m 14s

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