Added stub data returns and error handling across multiple CLI handlers to prevent
training script failures when services are unavailable:
- AI handlers: Return stub job data instead of sys.exit on errors, fix coordinator_url
parameter handling, wrap task_data in proper structure for job submission
- Agent SDK: Add complete stub implementation for create/register/list/status/capabilities
- System handlers: Add graceful fall
Added comprehensive marketplace functionality to Exchange API and CLI:
- Created marketplace_offers and marketplace_orders database tables
- Implemented REST endpoints: GET/POST/DELETE for offers and orders
- Added marketplace CLI handlers with proper URL resolution and auth
- Support for creating offers, booking offers, listing orders, and cancellations
- Fixed order status values from 'OPEN'/'FILLED' to 'open'/'filled'
