fix(config): canonicalise every AWS service alias in purchase suppressions, not only ec2/compute #1420

#1412 (fixed by #1419) made purchase suppressions match AWS EC2 recs whether they carry ec2 or the canonical compute slug. The same double-buy class exists for the other AWS alias pairs, which the provider treats as equivalent (providers/aws provider.go ~468-476) and which mapServiceSlug in internal/purchase/execution.go ~1268-1281 accepts:

• rds / relational-db
• elasticache / cache
• opensearch / search
• redshift / data-warehouse

So an API purchase sent as aws/relational-db writes a suppression under that slug, which misses the scheduler's aws/rds rec for the same capacity. That capacity can then be shown, and bought, again.

Proposed fix: generalise config.CanonicalSuppressionService (added in #1419) to map every AWS alias pair to one canonical value, ideally derived from the same table mapServiceSlug uses so the two can't drift. Add a test per pair in both directions, and keep Azure/GCP unchanged. Found by the independent review of #1419.