fix: add fare brand name to order change offer slices

[igorp1]

Summary

• add fare_brand_name to OrderChangeOfferSlice so order change offers expose the fare brand in the JS client
• update order change offer mocks and spec coverage to include the new field
• propagate the fixture update to related order change request and order change mocks used by type-checked tests

Testing

• ./node_modules/.bin/jest --runInBand src/booking/OrderChangeOffers/OrderChangeOffers.spec.ts
• ./node_modules/.bin/tsc --noEmit
• yarn test (passes as part of the attempted pre-commit hook)

Notes

• yarn build:test still fails in this environment during the existing Rollup step because rollup.config.mjs uses assert { type: 'json' }, which Node 22 rejects. The TypeScript portion completes successfully before that unrelated failure.

Linear Issue: TSP-1276