webhook: detect matchExpression selector overlaps

[sahitya-chandra]

Description

Port the selector-overlap helper downstream (for now) into the webhook package and use it from nodeSelectorsOverlap.

PR #212 fixed the common matchLabels subset case, but its implementation intentionally left a limitation for selectors using matchExpressions. This change replaces that heuristic with a local satisfiability check over selector requirements grouped by key, covering:

• In, NotIn, Equals, DoubleEquals, and NotEquals
• Exists and DoesNotExist
• numeric GreaterThan and LessThan
• non-subset matchLabels that still overlap on a possible node label set

Related Issue / PR

Follow-up to #212

Type of Change

/kind bug

Testing

• make setup-envtest
• env GOCACHE=/tmp/node-readiness-controller-go-cache make test

Checklist

• Code follows the project style and is formatted with go fmt
• Tests added for matchExpressions overlap and non-overlap cases
• make test passes

Does this PR introduce a user-facing change?

Fixed webhook validation to detect overlapping NodeReadinessRule node selectors that use matchExpressions, preventing additional conflicting rules from being accepted.

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[k8s-ci-robot]

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[sahitya-chandra]

/cc @ajaysundark