Fetch a payer by ID

Retrieves the full record for the payer identified by {id}. Use this once you already have a specific payer ID; use GET /payers to discover or browse payers.

Requires the READ_PAYER permission. The tenant-id header is required, but the payer catalog is not tenant-partitioned: any existing payer ID is returned regardless of which tenant-id value is supplied — the header is not used to authorize or scope the lookup, only for audit-trail attribution.

Returns a PayerResponse with id, externalPayerId, sourceId, and data (shaped per https://schemas.certifyos.com/entities/Payer.schema.json). This operation is read-only and has no side effects.