Create dimensional models for MITx Pro coupon/B2B ecommerce layer

What are the relevant tickets?

Part of epic #2072

Description

Two mart models cannot be migrated to the dimensional layer because the required dimensional abstractions do not yet exist:

• marts__mitxpro_all_coupons (#2099) — references 12 MITxPro-specific ecommerce int__ models including B2B orders, coupons, coupon payment versions, coupon products, and company data. No dimensional equivalent exists.
• marts__combined_discounts (#2081) — partially depends on marts__mitxpro_all_coupons and MITxOnline discount int__ models. Blocked by the coupon mart.

Scope

Investigate and create the dimensional models needed to support these marts. Candidates include:

• dim_coupon — coupon definitions, types, discount amounts across MITxOnline and MITxPro
• dim_b2b_contract (or extend dim_contract) — B2B company/contract-level order grouping
• tfact_coupon_redemption — coupon application events tied to orders

Source intermediates to consolidate:

• int__mitxpro__ecommerce_allcoupons, int__mitxpro__ecommerce_coupon, int__mitxpro__ecommerce_couponpaymentversion, int__mitxpro__ecommerce_couponproduct
• int__mitxpro__b2becommerce_b2border, int__mitxpro__b2becommerce_b2breceipt, int__mitxpro__ecommerce_company
• int__mitxonline__ecommerce_discount, int__mitxonline__ecommerce_discountproduct

Definition of Done

• New dimensional coupon/B2B models exist in models/dimensional/
• Models pass dbt tests
• Unblocks migration of #2081 (marts__combined_discounts) and #2099 (marts__mitxpro_all_coupons)

Additional Context

dim_discount and dim_discount_type already exist for MITxOnline discounts. The gap is specifically the MITxPro coupon/B2B layer which has no dimensional equivalent. Review existing dim_discount.sql and dim_contract.sql before designing new models.

Start by reviewing models/dimensional/dim_discount.sql and dim_contract.sql, then trace the listed MITxPro and MITxOnline intermediate models. Determine the dimensional coupon and B2B abstractions needed by #2081 and #2099. Run the dbt tests for the new models; done means the models exist in models/dimensional/ and unblock both mart migrations.