Add message_id field to Email Message Metadata (#333)

This change adds the message_id field to the email_message_metadata schema in the Unstable API specification. The message_id represents the unique identifier for email messages as specified in the Message-ID header.

Related to: intercom/intercom#451238

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Co-authored-by: Claude <[email protected]>

Author: marcmlc

descriptions/0/api.intercom.io.yaml

@@ -20091,6 +20091,11 @@ components:
           description: A list of an email address headers.
           items:
             "$ref": "#/components/schemas/email_address_header"
+        message_id:
+          type: string
+          nullable: true
+          description: The unique identifier for the email message as specified in the Message-ID header
+          example: "<[email protected]>"
     metadata:
           "$ref": "#/components/schemas/conversation_part_metadata"
     conversation_attribute_updated_by_workflow: