fix(purchase): wire the ElastiCache engine dedupe into the platform #1863

Split out of #682, whose GCP part is done (#1842 and #1848).

The platform pins the library's ElastiCache engine dedupe (pkg/recfilter duplicate checking over existing commitments) but never calls it, so it cannot suppress a repeat ElastiCache purchase. #682's plan scoped it out as a follow-up; it was not changed there.

To decide and do

1. Where it runs: the purchase path (execution), recommendation collection, or both. fix(purchase): refuse a GCP CUD when one for the same project, region and family started in the last 24 hours (Part of #682) #1848's GCP check runs per rec right before PurchaseCommitment, as a before-submission refusal, and is the model to compare with.
2. Whether a listing failure fails closed (as the GCP check does) or skips. The owner chose fail closed for GCP on fix(deps): adopt reviewed cloud-commitments-go purchase safeguards #452; confirm for ElastiCache.
3. The extra ElastiCache API calls per purchase, and the account and region scoping of the listing.
4. A test through the real path: a recent matching ElastiCache reservation refuses a repeat purchase and fails without the wiring.

Plan plus two review passes before code; this is a money path.

Part of #452.