Fix gRPC router workers never becoming ready with model set

[Bihan]

This PR fixes #4240.

Setting model without probes derives a POST {prefix}/chat/completionsprobe. ` A gRPC worker cannot answer HTTP at all, and the router answers 404 until workers are registered with it — but registration only considers ready jobs, so readiness waits on registration and vice versa.

Implements the third option from the issue: #4240

The third option tells: At minimum, do not apply the model default probe to non-router replica groups, and log when the router-worker sync returns early because no ready router job was found.

With this fix, in a service with a router group, the router is probed with GET /health and worker groups get no derived probe. Services without a router group keep the POST /v1/chat/completions probe.