IA-984 :Refactor: Fix endpoint URLs, convert static mapper to Spring bean, and harden education level resolution

Summary

This MR addresses several correctness and design issues across the assistant and user management services.

Changes

AssistantResolverService

• Fixed /children/current-user → /children/{userId} for user lookup endpoint
• Renamed findByEducationLevelAndType... → findFirstByEducationLevelAndType... to match updated repository method
• getEducationLevelFromUuid now throws FunctionalException instead of silently defaulting to CM1 on unresolvable levels

AssistantServiceImpl

• Replaced static AssistantMapper calls with injected AssistantMapper bean instance

AssistantMapper

• Converted from static utility class with a static ExternalApiClient field to a proper Spring @Component with constructor injection via @RequiredArgsConstructor

AssistantRepository

• Renamed findByEducationLevelAndTypeAndStatusAndIsActive → findFirstByEducationLevelAndTypeAndStatusAndIsActive to avoid ambiguity when multiple records match

ExternalApiClient

• Added Authorization header forwarding from the incoming request context
• Improved error handling: HttpClientErrorException logged at WARN without stack trace; unexpected errors also downgraded from ERROR to WARN

ExternalModelService

• Renamed config property MODEL_MANAGEMENT_API_BASE_URL → model-management.api.base-url to follow Spring property naming conventions

UserManagementServiceImpl

• Fixed /children/current-user → /children/{userId} for user lookup endpoint

Impact

• Fixes incorrect user resolution that was ignoring the userId parameter
• Prevents silent wrong-level assignment when UUID mapping fails
• Aligns property naming with Spring conventions