docs: Add previous_ticket_state_id to Ticket schema (Unstable) (#388)

Add previous_ticket_state_id field to Ticket response schema:
- Property definition in ticket schema
- Example value in response example
- Nullable string type (null when no state change history)
- Available in Unstable version only

Related to: intercom/intercom#474982

Co-authored-by: Claude Sonnet 4.5 (1M context) <noreply@anthropic.com>

Author: salmanbshah

descriptions/0/api.intercom.io.yaml

@@ -8526,6 +8526,7 @@ paths:
                       has_more: false
                     category: Customer
                     is_shared: true
+                    previous_ticket_state_id: '7490'
               schema:
                 "$ref": "#/components/schemas/ticket"
         '400':
@@ -23404,6 +23405,13 @@ components:
           type: boolean
           description: Whether or not the ticket is shared with the customer.
           example: true
+        previous_ticket_state_id:
+          type: string
+          nullable: true
+          description: The ID of the previous ticket state from the most recent state
+            change. Returns null if no state change history exists. Useful for tracking
+            state transitions for reporting and compliance.
+          example: '7493'
     ticket_deleted:
       title: Ticket Deleted
       type: object