Added messageReference to MESHInboxMessageReceived event

Author: tdroza-nhs

src/cloudevents/domains/digital-letters/2025-10-draft/data/digital-letters-mesh-inbox-message-received-data.schema.yaml

@@ -8,6 +8,9 @@ properties:
     $ref: ../defs/mesh.schema.yaml#/properties/meshMessageId
   senderId:
     $ref: ../defs/requests.schema.yaml#/properties/senderId
+  messageReference:
+    $ref: ../defs/requests.schema.yaml#/properties/messageReference
 required:
   - meshMessageId
   - senderId
+  - messageReference