[Bugfix][Router] Remove ghost endpoint on evicted-worker MODIFIED event

[jiawen7777]

When a worker pod is evicted under node pressure, the kubelet clears its podIP and Kubernetes emits a MODIFIED event with engine_ip=None. On current main the MODIFIED branch returns early on a None IP, so the already-registered endpoint lingers as a stale ("ghost") backend. The router keeps routing requests to that address even though no worker is behind it, and those requests hang until they time out. The endpoint is only cleared when the router process restarts.

An empty IP on MODIFIED is ambiguous: a Pending pod has no IP yet (skip it), but an Evicted pod has had its podIP cleared and its endpoint must be removed. Remove the registered engine on this path, mirroring the DELETED handling, and log a WARNING so the removal is observable. This fix has been verified to resolve the problem in real deployment.

Scope: K8sPodIPServiceDiscovery only; ADDED and graceful-drain paths unchanged; no routing changes. Adds 5 regression tests.

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FIX #1008

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[gemini-code-assist]

Code Review

This pull request addresses an issue where evicted Kubernetes pods with cleared pod IPs could linger as stale ('ghost') endpoints in the service discovery. It updates the _on_engine_update method in K8sPodIPServiceDiscovery to check if a modified pod with a None IP is already registered, and if so, removes it. Additionally, a comprehensive set of unit tests has been added to verify this behavior and ensure regression coverage. I have no feedback to provide on these changes.