Add health check endpoint to App Proxy component

[HyeockJinKim]

Description

Implement a /health endpoint for the App Proxy component (both coordinator and worker) to provide health status information.

Acceptance Criteria

• Add /health endpoint to App Proxy coordinator REST API
• Add /health endpoint to App Proxy worker REST API
• Return component status (healthy/unhealthy)
• Include proxy service status
• Include backend connectivity status
• Include basic metadata (version, uptime, etc.)
• Return HTTP 200 for healthy state
• Return HTTP 503 for unhealthy state
• Add unit tests for both endpoints
• Update API documentation

Technical Details

• Path: /health
• Method: GET
• No authentication required for basic health check
• Should verify proxy functionality and backend connectivity
• Response format should be consistent with other components
• Both coordinator and worker should have separate health endpoints

JIRA Issue: BA-2429