dasBidAdapter: add server-driven adserverTargeting support

[skoklowski]

Type of change

• Bugfix

• Feature

• New bidder adapter

• Updated bidder adapter

• Code style update (formatting, local variables)

• Refactoring (no functional changes, no api changes)

• Build related changes

• CI related changes

• Does this change affect user-facing APIs or examples documented on http://prebid.org?

• Other

Description of change

This change adds support for passing custom targeting key-values from the DAS server response to GAM (Google Ad Manager).

Changes:

• Extract targeting object from bid.ext.targeting in the interpretResponse function
• Pass the entire targeting object as adserverTargeting on the bid response
• Added unit tests covering presence, absence, and multiple targeting keys

How it works:

The DAS server can now control which key-values are sent to GAM without requiring changes to the Prebid.js adapter. When the server returns a targeting object in the bid extension, all keys are automatically passed to GAM targeting.

Since these are custom keys (without hb_ prefix), Prebid.js will aggregate values from all DAS bids for each ad unit, not just the winning bid.

Example bid response from server:

{
  "seatbid": [{
    "bid": [{
      "impid": "bid123",
      "price": 3.5,
      "ext": {
        "targeting": {
          "bidder_variant": "variant_a",
          "custom_key": "custom_value"
        }
      }
    }]
  }]
}