fix: validate taint value format with CEL

[DsThakurRawat]

Description

The CEL validation rules on spec.taint validated the key format and capped value length at 63 characters, but did not validate the character set of value. Because the API server enforces label value syntax on node taints ((([A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9])?), a rule with an invalid taint value (such as spaces or invalid punctuation) was admitted at CRD validation, but every subsequent node patch failed with Invalid from the API server, landing the node in status.failedNodes with EvaluationError. Because taint.value is immutable, the rule could not be repaired in-place and had to be deleted and recreated.

This adds a CEL validation rule on spec.taint matching the standard Kubernetes label value character set:

rule: '!has(self.value) || self.value.matches(''^(?:(?:[A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9])?$'')'

Empty or omitted values continue to be permitted as before.

Related Issue

Fixes #467

Type of Change

/kind bug

Testing

• Added unit tests in internal/controller/nodereadinessrule_controller_test.go under CEL Validation for taint value format verifying rejection of invalid taint values and acceptance of valid/empty values.
• Added e2e test case in test/e2e/taint_validation_test.go.
• Verified make manifests, make test (exit 0, controller 86.3%), and make lint (0 issues).

This PR was written in part with the assistance of generative AI; the failure was reproduced against envtest and the change was reviewed and tested by hand.

Checklist

• make test passes
• make lint passes

Does this PR introduce a user-facing change?

Reject NodeReadinessRules with invalid taint values at admission time rather than failing subsequent node patches with API server validation errors.

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[kubernetes-prow]

Hi @DsThakurRawat. Thanks for your PR.

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