Add comprehensive tests for agent scaling group update

## Objective

Add comprehensive test coverage for the agent scaling group update functionality, including validation, service layer, and end-to-end flows.

## Test Coverage Scope

### Unit Tests

- Validation logic: prevent update when active kernels exist
- Service layer: update_scaling_group() business logic
- Repository layer: database operations
- Error handling and edge cases

### Integration Tests

- End-to-end GraphQL mutation with active kernels (should fail)
- End-to-end GraphQL mutation without active kernels (should succeed)
- RPC communication with agent
- Transaction rollback scenarios

### Test Scenarios

- Agent with no kernels: update should succeed
- Agent with active kernels: update should fail with clear error message
- Agent with terminated kernels: update should succeed
- Multiple concurrent update attempts
- RPC failure handling
- Database transaction failures

## Dependencies

- Requires BA-3570, BA-3571, and BA-3572 to be completed

## Expected Outcome

All new code paths have appropriate test coverage, ensuring the validation and refactoring work correctly in various scenarios.

JIRA Issue: BA-3573