Generate MerchantAggregate IDs through IdGenerationService #2000

Problem

MerchantDomainService.CreateMerchant uses a caller-provided MerchantId or Guid.NewGuid(). This makes new IDs random and bypasses IdGenerationService.

Suggested ID inputs

Use EstateId plus a stable upstream merchant identifier or merchant number. Keep a caller-provided ID as the stable key where it is part of the integration contract. Merchant name is not a safe key because it is neither guaranteed unique nor immutable.

Acceptance criteria

• Add a deterministic merchant ID generator to IdGenerationService based on the agreed stable identifier and estate scope.
• Use it consistently for creation and subsequent lookups.
• Define compatibility for existing streams and client-supplied merchant IDs.
• Cover retries, same external key in different estates, and duplicate keys within an estate.