AdCP 3.3: expected_confirmation_at on submitted buys and proposals #1640

Spec

expected_confirmation_at can appear on the submitted envelope of create_media_buy, buy_products and accept_proposal; on a submitted finalize-only refine_proposals (including extend_hold); and on get_media_buys while confirmed_at is null. Sellers whose confirmation waits on a scheduled event SHOULD set it. Buyers SHOULD check status at that time instead of polling tightly. It's an estimate. On buy_products, accept_proposal and refine_proposals it's sent only to callers negotiating 3.3.

What's new §Expected confirmation time; create_media_buy §Expected confirmation time (tag v3.3.0-beta.0).

Work (after regen)

• Buyer: task polling and continuations (TaskResult, wait_for_completion-style helpers, purchase continuations) schedule the next status check from expected_confirmation_at when present, with a bounded floor and ceiling. Surface it on results.
• Seller: a framework helper to set it on submitted responses; drop it on the gated tasks for callers below 3.3. Check the generic version projection already does this once _v32 bases exist.
• Tests.

Batch: C (media-buy flow; needs regen).