fix: handle nested assortment expansion and type narrowing

[MonsterDeveloper]

Summary

• Omit unsupported nested expand keys from each assortment union member, so Product, Service, and Bundle do not acquire a required product: never field.
• Let isAssortmentOfType accept assortments by their meta.type discriminator, including demand positions loaded with fields: ["stock"].
• Add type tests for both regressions.

Checks

• bun check
• bun typecheck
• bun run test