Replace dynamic MODEL_SERVICE_DEPENDENCIES injection with frozen dataclasses
(CivitAISearchDependencies, CustomUrlDependencies, ModelDetailsDependencies)
to make service dependencies explicit and type-safe. Update model services to
construct via RouteContext, and add tests enforcing minimal dataclass fields.
Break down the monolithic model_service into smaller, single-responsibility
services: CivitAISearchService, CustomUrlService, and ModelDetailsService.
Introduce ModelServiceDependencies base class to centralize RouteContext
dependency injection. Update model_service to act as a thin facade that
delegates to the specialized services. Add unit tests validating the new
service boundaries and basic request validation behavior.
Dariusz L