Update ContractServiceTest to use mockito instead of wiremock

Tech-debt: Currently this test class has both a setup of mockito for the partnerApi and also includes wiremock. The mockito partnerApi is not being used by the tests but should be since it will make testing specific data possible for unit tests. 

Done:

In https://github.com/RedHatInsights/rhsm-subscriptions/blob/main/swatch-contracts/src/test/java/com/redhat/swatch/contract/service/ContractServiceTest.java

• Remove:

  @QuarkusTestResource(value = WireMockResource.class, restrictToAnnotatedClass = true)

• Create partnerApi mock that is used by tests