fix(aws/savingsplans): add USD currency + region filter and deterministic sort to findOfferingID #1066

Problem

findOfferingID in providers/aws/services/savingsplans/client.go calls
DescribeSavingsPlansOfferings without narrowing by currency or region, and
then returns the first result from an unordered API response. AWS does not
guarantee ordering of DescribeSavingsPlansOfferings results, so when
multiple offerings match (e.g. USD vs CNY, or global vs region-scoped) the
chosen offering ID is nondeterministic and may be non-USD or off-target.

Source: finding 08-L1 (SP portion) from the code-review series (report 08).

Evidence

DescribeSavingsPlansOfferingsInput already accepts:

• Currencies []types.CurrencyCode — lets us pin to USD server-side.
• Filters []types.SavingsPlanOfferingFilterElement with
  SavingsPlanOfferingFilterAttributeRegion — lets us pin to the client
  region for EC2Instance SPs (Compute/SageMaker/Database SPs are
  region-agnostic; filter is omitted for them).

Currently neither filter is set, so any currency or region variant can
land in position 0 of the response.

Fix

1. Set Currencies: []types.CurrencyCode{types.CurrencyCodeUsd} on the
   input before calling DescribeSavingsPlansOfferings.
2. For EC2Instance SPs, add a region filter element (the only SP type that
   has region-specific offerings). Compute, SageMaker, and Database SPs are
   global.
3. Sort SearchResults by OfferingId (lexicographically) before returning
   the first match, so the tie-break is stable if multiple offerings still
   match after filtering.

Files

providers/aws/services/savingsplans/client.go,
providers/aws/services/savingsplans/client_test.go

PR

Folds into #1039 (fix/aws-sp-pagination), which already modifies this file.