feat: add catalog discount previews

[ShuoRen-TT]

Summary

• extend dev.ucp.shopping.discount to Catalog Lookup
• expose discounts.available only on get_product responses
• reuse the available_discount model introduced by feat: add discoverable available discounts to discount extension #609
• keep Cart and Checkout authoritative for eligibility and application

Proposal and dependency

Draft implementation for #707.

This PR is intentionally stacked on #609 and includes its current commit so the Catalog composition can reference available_discount. It should not merge until:

• [Proposal]: Extend discoverable discounts to catalog product details #707 is accepted as Provisional by the Tech Council
• feat: add discoverable available discounts to discount extension #609 is merged
• this branch is rebased onto main so the final diff contains only the Catalog-specific changes

Scope

• no changes to the core Catalog schema
• no changes to Catalog Search or batch Lookup
• no discount submission from get_product
• no discounts.codes or discounts.applied in Catalog
• no applied or remaining_amount in Catalog preview entries
• no private discount code exposure

Validation

• ucp-schema 1.4.1 lint source/
• pre-commit run --files docs/specification/discount.md source/schemas/shopping/discount.json
• python scripts/validate_examples.py --schema-base source/schemas/ (294 passed)
• python scripts/test_validate_examples.py (50 passed)
• strict composed-schema checks for Catalog get_product and batch lookup
• bash scripts/build_local.sh --draft-only

Refs #707
Depends on #609