As a buyer, I want the first bid request to include IAB cohorts, signals, and synthetic ID #269

Story

As a buyer,
I want the first bid request to include IAB cohorts, publisher signals, and the TS synthetic ID,
so that even before ID sync, I have meaningful data to inform my bidding and can initiate the sync process.

Parent Epic: #265

Acceptance Criteria

• Every bid request includes the TS synthetic ID (already implemented via user.id)
• If IAB audience taxonomy cohorts are available, they are included in the request
• If publisher-defined signals are available, they are included in the request
• This data is present regardless of whether a buyer ID mapping exists

Open Questions

• What is the source of IAB cohorts? (Publisher-provided? TS-computed? Third-party integration?)
• What signal format? (IAB Seller Defined Audiences? Custom key-values?)
• Where in the OpenRTB request should cohorts/signals go? (user.data? site.content.data?)