[BUG] Webhook Health Checker Panics on Startup, Crashes Controller

[sagar-h007]

What happened:

The kruise-controller webhook server crashes on startup with an unrecoverable panic if:

1. The CA certificate file cannot be loaded from /kruise/cert-dir/ca-cert.pem, OR
2. The filesystem watcher (fsnotify.Watcher) fails to initialize, OR
3. The watcher fails to add the CA cert file path

The panic occurs in pkg/webhook/util/health/checker.go at lines 113, 117, and 120 within a sync.Once block, making it impossible to recover.

What you expected to happen:

The health checker should:

1. Return an error instead of panicking
2. Allow the webhook server to start with a failing health check
3. Log the error and retry initialization gracefully
4. Not crash the entire controller process

How to reproduce it (as minimally and precisely as possible):

1. Deploy kruise-controller in a namespace where the cert directory is misconfigured or missing
2. OR: Deploy on a filesystem that doesn't support fsnotify (certain network filesystems)
3. OR: Set incorrect permissions on the certificate directory
4. Observe the controller pod crash-looping with panic trace like:

[sagar-h007]

/assign