Preserve request-scoped Prebid ad units across GPT refreshes

[ChristianPavilonis]

Description

The managed Prebid integration can lose publisher bidder parameters and banner zone metadata when handling GPT refreshes.

Publishers may pass ad units only through pbjs.requestBids({ adUnits }), leaving pbjs.adUnits empty. The integration currently folds those request-scoped bidder definitions into the trustedServer bid before dispatch. When a later GPT refresh reconstructs an ad unit from pbjs.adUnits, it can therefore send a new /auction request with empty bidderParams and no zone.

A related case occurs when a publisher calls googletag.pubads().refresh() synchronously from bidsBackHandler to deliver the auction result. Treating that delivery refresh as a new independent auction produces an immediate duplicate request with incomplete ad-unit data.

Expected behavior

• Preserve immutable request-scoped publisher ad-unit metadata before folding bids into trustedServer.
• Do not treat a synchronous GPT refresh used to deliver a completed publisher auction as a new auction.
• Reconstruct later independent refresh auctions with the original bidder parameters, client bids, and banner zone.
• Prevent synthetic refresh units from overwriting valid publisher snapshots.
• Continue handling malformed or missing bids arrays without throwing.

Reproduction outline

1. Keep pbjs.adUnits empty.
2. Call pbjs.requestBids({ adUnits, bidsBackHandler }) with bidder params and a banner zone supplied only in the request-scoped ad unit.
3. Call GPT refresh synchronously from bidsBackHandler.
4. Observe an immediate second /auction request with empty bidder params and missing zone.
5. Trigger a later independent GPT refresh and observe the same missing metadata.

Acceptance criteria

• The publisher auction produces one server auction request, with no immediate delivery-refresh duplicate.
• A later independent GPT refresh starts one auction and retains the original bidder params and zone.
• Nested bidder-param objects are isolated from later mutation.
• Regression tests cover malformed bids, synthetic-unit recursion, synchronous delivery refreshes, and deferred independent refreshes.

[ChristianPavilonis]

Implementation is in #912, stacked on #893 (add-prebid-error-diagnostics).

[ChristianPavilonis]

Follow-up live tracing found that the publisher sets GPT targeting inside bidsBackHandler, but executes the matching explicit GPT refresh 68–325 ms after the callback returns. #912 now retains a code-scoped, one-shot delivery context only after a matching targeting signal, with a 1-second expiry. It also permits GAM-only extras in the same explicit delivery list. Replaying the rebuilt bundle produced no immediate duplicate, while a later independent refresh still auctioned with complete params and zone.