[router] defer health checking to router init

[ByronHsu]

Motivation

Defer the initial health waiting to the router, so in sglang_router.launch_router, it knows to launch router only after all workers are healthy

Modifications

Checklist

• Format your code according to the Contributor Guide.
• Add unit tests as outlined in the Contributor Guide.
• Update documentation as needed, including docstrings or example tutorials.