Update App Proxy routing with traffic_status filter

## Overview

Update App Proxy routing logic to filter routes by both health status and traffic status.

## Tasks

1. Update App Proxy coordinator route query:
- Filter: status=HEALTHY AND traffic_status=ACTIVE
- Exclude STANDBY routes (not ready for traffic)
- Exclude DRAINING routes (shutting down)
1. Update health checker to handle traffic_status
1. Update route registration/deregistration logic
1. Write integration tests

## Routing Condition

```python
route.status == RouteStatus.HEALTHY AND route.traffic_status == RouteTrafficStatus.ACTIVE
```

## Reference

- Design doc: docs/endpoint-model-restructure-TODO.md
- App Proxy code: src/ai/backend/appproxy/coordinator/

JIRA Issue: BA-3438