Convert Contact.owner_id to string in unstable API version

Aligns Contact.owner_id field with Admin.id type (both now strings in Unstable version).
Existing public versions (2.11-2.15) maintain integer type via versioning transformations.

Updates Preview spec (descriptions/0/api.intercom.io.yaml):
- Change Contact schema owner_id from type: integer to type: string
- Update example from 123 to '123'

This matches PR intercom/intercom#499855.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: dan0505

descriptions/0/api.intercom.io.yaml

@@ -20271,11 +20271,11 @@ components:
           description: The contacts name.
           example: John Doe
         owner_id:
-          type: integer
+          type: string
           nullable: true
           description: The id of an admin that has been assigned account ownership
             of the contact.
-          example: 123
+          example: '123'
         has_hard_bounced:
           type: boolean
           description: Whether the contact has had an email sent to them hard bounce.