feat: define recipient-required gift checkout

[seungy0]

Summary

Adds a Checkout-only Gift extension for purchases sent to another person.

A Platform requests this flow with:

{
  "gift": {
    "recipient_required": true
  }
}

gift: {} is a no-op. recipient_required: true means the Business must have a selected, eligible recipient before it accepts Complete Checkout. It does not define how the recipient is selected or where that UI is shown.

The Business keeps the selection as private state bound to the Checkout. gift.recipient is response-only and contains only a buyer-facing display name:

{
  "gift": {
    "recipient_required": true,
    "recipient": {
      "display": {
        "name": "Mom"
      }
    }
  }
}

This extension does not expose or accept recipient IDs, selection tokens, friend lists, contact data, or addresses.

Flow

• If buyer interaction is needed, the Business uses the existing escalation flow and continue_url.
• The buyer may log in and choose a recipient in the Business experience. Returning from that experience is not proof that a selection was made; the Platform reads the Checkout again.
• An Update must include recipient_required: true to retain the Gift request. Omitting it clears the Business private recipient selection.
• Complete omits gift. The Business revalidates and freezes the selection before creating payment or order effects.

Scope

This does not define recipient search, reusable recipient handles, multi-recipient checkout, messages or cards, acceptance, scheduling, or recipient data in Orders or events.

[damaz91]

Hello @seungy0, thanks for your contribution to UCP. This does not seem just a documentation PR as the title would suggest. Seems like you are trying to propose a new extension.

This probably would entail a schema change as well. Please add it in the PR so we can move forward with the review.

Thanks.