Expose the inbound email Message-ID that customers send, so it can be reused in notifications, automation rules, and via REST APIs

JSM already stores this header internally for email threading, but it is not currently visible or accessible

Having it available as a read-only field/smart value would let us reliably correlate JSM requests with original emails in external systems, improve troubleshooting of mail flows, and include this identifier in custom notification templates without fragile email-gateway workarounds.