Apply service/repository layer to agent scaling group update

## Objective

Refactor agent scaling group update logic to use service/repository pattern, improving separation of concerns and testability.

## Current Issues

- Business logic mixed with GraphQL resolver
- Direct DB updates without repository layer
- Validation logic (from BA-3570) needs a proper home

## Implementation Scope

- Create AgentService.update_scaling_group() method
- Apply Action/ActionResult pattern (UpdateAgentScalingGroupAction, UpdateAgentScalingGroupActionResult)
- Integrate BA-3570 validation logic into service layer
- Use AgentRepository for DB updates
- Improve error handling and logging

## Expected Outcome

Business logic moves from GraphQL resolver to service layer, following the established service/repository pattern used by other components.

JIRA Issue: BA-3571