refactor(aws): deduplicate resolveTag across 4 service clients into a shared helper #1390

#1389 added an identical resolveTag(execID string) string helper (returns execID or "no-exec") to each of 4 service packages (redshift, opensearch, memorydb, elasticache) to drop them under gocyclo 10. That's 4 copies of the same trivial function. Consolidate into one shared helper.

All 4 services already import providers/aws/internal/purchasecfg (verified), so moving resolveTag there (or into providers/aws/internal/tagging if more semantically fitting for a client-token/idempotency tag) needs no new imports and no cycle. Delete the 4 local copies, call the shared one. Behavior identical; existing tests unchanged and green; gocyclo must stay <=10 on the callers. Owner-requested during the merge-driver.