Part of #4084 - FastAPI Router Migration Proposal

Scope

Migrate the agents API from @webmethod decorators to standard FastAPI routers.

Tasks

• Extract Protocol to agents/service.py (remove @webmethod)
• Create agents/models.py with Pydantic request/response models
• Create agents/routes.py with FastAPI router
• Register router in router registry
• Update agent provider implementations
• Update tests

Dependencies

• Requires Phase 1 infrastructure (FastAPI Router Migration Proposal #4084)

Related to #4084