refactor(aws): consolidate 5 near-identical RI term parsers (drift already started)

[cristim]

Duplication audit (owner-requested) found LeanerCloud/cloud-commitments-cli#1207 added 5 near-identical term-parsing helpers across the service clients, and DRIFT HAS ALREADY BEGUN: requiredMonthsForTerm (opensearch + redshift, identical bar the service name in the error), getDurationString (elasticache + rds, identical), getDurationStringForAPI (memorydb, a variant that ALSO accepts "12"/"36"). All validate the same 1yr/1/3yr/3 domain on a MONEY path (RI term -> offering purchase), so divergence risks mispriced/mistermed purchases.

Consolidate into a shared ParseTermMonths(term, service string) (int, error) + duration-string formatter in providers/aws/internal/purchasecfg (alongside LeanerCloud/cloud-commitments-cli#1390's ResolveTag) or a sibling terms pkg. Keep each caller's fail-loud behavior (LeanerCloud/cloud-commitments-cli#1207); reconcile the memorydb "12"/"36" variant deliberately (decide if numeric forms are valid everywhere). All existing term tests must pass unchanged.

[cristim]

Claimed by worker 4 (shard rank 24). Starting consolidation of the five RI term parsers into a shared helper in providers/aws, per the issue's fix direction.

[cristim]

Resolved by PR #273, merged normally as 5599b24 from independently reviewed head 60868b6. All nine owned source/test files match the verified head byte-for-byte. Shared term parsing, six service consumers, MemoryDB legacy aliases and complete diagnostic, and exported duration constants are preserved. Local Go 1.26.6 builds and the full AWS race suite passed; pre-merge CI is green. Actual merge-commit CI is being watched. Fixture and mock evidence was used; no live cloud purchase was exercised.