Remove duplicate validateSpawnRequest from AgentsController. Validation
is now handled exclusively by:
1. ValidationPipe + DTO decorators (HTTP layer, class-validator)
2. AgentSpawnerService.validateSpawnRequest (business logic layer)
This eliminates the maintenance burden and divergence risk of having
identical validation in two places. Controller tests for the removed
duplicate validation are also removed since they are fully covered by
the service tests and DTO validation decorators.
Co-Authored-By: Claude Opus 4.6 <[email protected]>
