fix: webhook fails closed when rule listing errors

[dorodb-web22]

Fixes #251

When validateTaintConflicts cannot list existing rules, it previously logged the error and allowed the request through (fail-open).
This means two rules with the same taint key/effect and overlapping selectors could be created during an API degradation window, causing taints to flip-flop indefinitely on affected nodes.

This change returns an InternalError instead, rejecting the request with a clear message so the client can retry when the system is healthy.
This aligns with the Kubernetes admission webhook best-practice offail-closed validation.

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[AvineshTripathi]

I'm not sure about using field.NewPath("spec", "taint", "key") here. A List failure is a systemic/infrastructure issue, not an error with the user's input in that specific field. Attributing it to the taint key could be misleading for users debugging their YAML.