fix: stop exporting stale metrics for completed or failed pods

[maishivamhoo123]

Enhance pod validation to check both pod phase and container running status.

Previously, volcano-vgpu-monitor would export metrics for completed or failed
pods as long as the Pod object existed in Kubernetes. This caused stale metrics
to remain visible in Prometheus.

Changes:

• Verify pod is in PodRunning phase before considering it valid
• Check that target container is actively running (not terminated or waiting)
• Parse cache directory names to safely extract pod UID and container name
• Add detailed logging for validation results
• Preserve existing 300-second grace period for cache cleanup

Fixes #130

[hami-robot]

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[archlitchi]

we shouldn't validate the container status, because if container crashes, the folder will be deleted, that will make any attempt to restart the container failed