[cristim]

Refs #53. Follows #611 (EC2) and #615 (RDS); mirrors their semantics for MemoryDB, OpenSearch and Redshift (one commit per service).

PurchaseCommitment reads the resolved offering back by ID and refuses when upfront (FixedPrice x count) exceeds the approved CommitmentCost by more than 5%, when the rec or offering currency is non-USD (case-insensitive, empty means USD), or when the read fails, returns other than exactly one offering, or has no price. Zero approved cost and no-upfront purchases are not checked. The guard runs after the idempotency dedupe, so a re-drive of an existing commitment still returns ExistingCommitment success (Redshift: after guardAgainstRepurchase).

Units, checked against the aws-sdk-go-v2 types:

• MemoryDB ReservedNodesOffering.FixedPrice float64 (memorydb@v1.31.4 types.go:576), per node (existing code multiplies by NodeCount). No currency field, so only the rec currency is checked, and there is no nil case (a missing price reads as 0).
• OpenSearch ReservedInstanceOffering.FixedPrice *float64 (opensearch@v1.52.3 types.go:2372) with CurrencyCode *string (:2364), per instance (purchase takes InstanceCount).
• Redshift ReservedNodeOffering.FixedPrice *float64 (redshift@v1.58.3 types.go:1791) with CurrencyCode *string (:1784), per node (existing code multiplies by NodeCount).
• Cost Explorer UpfrontCost is documented as "How much purchasing this instance costs you upfront" (costexplorer@v1.63.1 types.go:1971); it is read as the total for the recommended count, the same assumption as the EC2 and RDS guards.

Tests per service cover: above +5% refused with no purchase, exactly +5% buys, lowercase usd buys, EUR on rec (and offering where the field exists) refused, zero cost unchecked, no-upfront unaffected, re-drive after a price rise succeeds, describe error, two and zero offerings, nil price (OpenSearch, Redshift). 41 mutants in a scratch copy were all killed. go test -race -count=1 ./... passes in providers/aws with GOWORK=off and with the workspace; golangci-lint v2.10.1 reports 0 issues. PurchaseCommitment complexity was kept under the gocyclo limit by extracting reservationNameFor (OpenSearch) and reservedNodeTotal (Redshift).

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • MemoryDB, OpenSearch, and Redshift purchases are blocked when upfront costs exceed the approved amount by more than 5%, currency is unsupported, or the price cannot be verified.
  • MemoryDB purchases with upfront costs are blocked when the offering is zero-priced or the purchase is in a China region.
  • Non-finite approved upfront amounts are rejected across EC2, RDS, MemoryDB, OpenSearch, and Redshift.
  • Purchases at the 5% limit remain allowed. Existing reservations, zero approved upfront amounts, and purchases without upfront costs remain unaffected.