fix(aws): payment-option/engine/term correctness gaps in AWS RI service clients #5

Problem (money path)

Adversarial review of providers/aws/ (report docs/code-review/08-provider-aws.md)
surfaced three HIGH-severity correctness gaps in the AWS RI service clients that
sit directly on the purchase / money-moving path. None were captured by the
existing tracking issues (LeanerCloud/cloud-commitments-cli#1012–LeanerCloud/cloud-commitments-cli#1035 only filed 08-C1 and 08-H1).

08-H2 — Redshift offering selection ignores the requested payment option

• providers/aws/services/redshift/client.go (matchesOfferingType, findOfferingID)
• matchesOfferingType returns true for any offering whose
  ReservedNodeOfferingType is Regular/Upgradable and discards the
  paymentOption argument entirely. So a no-upfront recommendation can be
  filled with an all-upfront offering (or vice-versa) — a different price and
  cash-flow profile than the operator approved. The OnDemand/savings math in the
  UI no longer matches what was bought.
• Fix: select the offering whose price shape matches the requested payment option
  (Redshift encodes payment via FixedPrice + RecurringCharges, not the
  Regular/Upgradable enum): all-upfront = upfront>0 & recurring==0;
  no-upfront = upfront==0 & recurring>0; partial-upfront = upfront>0 &
  recurring>0. Surface the chosen payment option in GetOfferingDetails.

08-H3 — ElastiCache silently defaults an unknown payment option to Partial Upfront

• providers/aws/services/elasticache/client.go (convertPaymentOption)
• The default branch returns "Partial Upfront" (no error, no log). An empty
  or malformed rec.PaymentOption therefore buys a Partial-Upfront reservation
  rather than failing. RDS's equivalent (services/rds/client.go) correctly
  returns an error for unknown options.
• Fix: mirror RDS — return (string, error) and propagate the error up through
  findOfferingID / PurchaseCommitment.

08-H4 — Per-service throttle can swallow a fully-throttled service as "no recs"

• providers/aws/recommendations/client.go (GetRecommendationsForService,
  mergeServiceResults, GetAllRecommendations)
• The (term × payment) sweep tolerates per-combo errors and only fails when
  successCount == 0. A sustained throttle makes every combo fail; the wrapped
  lastErr then reaches mergeServiceResults, which logs it at WARN and drops
  that whole service's recommendations, returning the others as if the run
  succeeded. A throttled run is indistinguishable from "no savings available".
• Fix: distinguish "no recommendations" from "all attempts errored" at the
  GetAllRecommendations level so the caller can tell a fully-failed service
  from an empty one.

Files

• providers/aws/services/redshift/client.go
• providers/aws/services/elasticache/client.go
• providers/aws/recommendations/client.go

Findings

08-H2, 08-H3, 08-H4 (the untracked HIGH set from report 08).

Scope note: report 08 also lists Medium/Low/Nit AWS findings (08-M2..M7,
08-L2/L4/L5, 08-N1/N5). Those are deferred to a follow-up PR to keep this one
on the four-High money-path fixes and under the ~400-line review budget, per
the split guidance in 16-remaining-findings-plan.md.