test(capital): add DynamicOffersApi test coverage

- Add DynamicOffersApiTest covering all three endpoints (getAllDynamicOffers,
  calculatePreliminaryOfferFromDynamicOffer, createStaticOfferFromDynamicOffer), including
  RequestOptions and missing-id validation variants, with corresponding mock JSON fixtures.
- Also fix GrantOffersApiTest to use isNull() instead of eq(null) alongside ArgumentCaptor,
  consistent with Mockito matcher rules.