Endpoint Testing: GetZoningDistrictClassCategoryColors, e2e and service unit

[TangoYankee]

Description

Write e2e tests and service unit tests for the GetZoningDistrictClassCategoryColors endpoint.

Acceptance criteria

• e2e tests
  • [x] 200 path
  • [x] 500 path from database error
• service unit tests
  • [x] successfully return expected data

Related tickers

blocked by #151, which will create the required zoning-district-class repository mock and zoning-district-class entity schema