Get payer details (FHIR Aggregator or FhirPayer)

Validates ClientId and XApiKey against FHIR.Clients. The client must have ClientType FhirAggregator or FhirPayer. FhirProvider clients receive 400 with "One of ProviderTaxId, VendorId, or GlobalProviderId is required" (use GET /v1/internal/provider). Returns the first active ClientMappings row (lowest Id): PayerId and Validation.PayerID are AgencyId (payer/agency in HHA), and Environment is derived from the mapping Env. BaseUrl is always null.