Deployment revision update causes downtime by terminating old route before new route is ready

## Background

When a deployment is updated (e.g., model service configuration change) and a new revision is created, the expected behavior is:

1. New revision's route is created and becomes ready
2. Old revision's route is terminated (blue-green style)

## Current behavior:

1. Old revision's route is terminated first
2. New revision's route is created afterward

This causes downtime during deployment updates — the service is unavailable between the old route termination and the new route becoming ready.

## Success Criteria

- [ ] Deployment update creates new revision route before terminating old revision route

- [ ] No downtime window during revision transition

- [ ] Rollback scenario (new revision fails to start) does not leave service without any active route

JIRA Issue: BA-5743