feat: add discoverable available discounts to discount extension

[hemanth]

Closes #346

Summary

Adds an available array to the discount extension's discounts_object, allowing businesses to surface discoverable promotions before the buyer applies a code.

Changes

• source/schemas/shopping/discount.json:
  • Added available_discount definition with id, title, type (automatic/code), conditions, applied, and remaining_amount
  • Added available array to discounts_object (response-only via ucp_request: omit)

Use cases

• Agent tells buyer: "There's a 15% off summer sale running"
• Agent prompts: "Add $12 more for free shipping"
• Automatic discounts surfaced so the buyer knows they're getting a deal
• Code-based offers hint at existence without exposing the code

Design decisions

• Business-optional: merchants that don't want to expose promotions just omit the field
• Response-only (ucp_request: omit) — this is purely informational from business to platform
• type: code exposes the existence of a promotion but NOT the code itself
• remaining_amount enables threshold-based prompts ("add $X more for...")