MLC-297/dynamic search & approve/reject for unregistered agents and introduce SpecBuilder
- add POST /approve and POST /reject/{id} endpoints in UnregisteredAgentController
- add GET /unregistered-agents (paged + filters) for listing all UnregisteredAgents
- introduce SpecBuilder for conditional Specification chains
- create search-DTO hierarchy: BaseSearchRequest, UserSearchRequest, AgentSearchRequest, ClientSearchRequest, UnregisteredAgentSearchRequest
- refactor list methods for clients, agents, and unregistered agents to use SpecBuilder and search DTO hierarchy