As a buyer, I want the prebid request to include my buyer ID if one has been synced #271

Story

As a buyer,
I want the prebid bid request to include my buyer ID if one has been synced,
so that my bidding system can match the user to my internal profiles.

Parent Epic: #265

Acceptance Criteria

• If a buyer ID exists in the KV store for the current synthetic ID and bidder, it is included in the bid request
• The ID is passed using the IAB user.eids standard so Prebid Server routes it correctly
• If no buyer ID exists for a given bidder, that bidder's request is sent without a buyer UID (normal behavior)

Note: This story is closely related to #267 but from the buyer's perspective. #267 covers TS-side enrichment logic; this story ensures the buyer receives the ID in the request they receive from Prebid Server.