fix(controller): emit Warning events on failure paths for taint operations and reconciler errors

[Sreejesh06]

Description

This PR adds Warning event emissions across operational failure paths for taint additions/removals and rule reconciliation errors, ensuring failures are surfaced to operators via standard Kubernetes events.

To align with the observability design in #344 and prevent event flooding on the rule custom resource:

• Node-Scoped Failures (e.g. AddTaintError, RemoveTaintError): Emitted only on the affected Node object, referencing the rule as the relatedObject.
• Rule-Scoped Failures (e.g. ListNodesError, StatusUpdateError, FinalizerError): Emitted on the NodeReadinessRule object itself.

This ensures running kubectl describe node properly surfaces scheduling or taint application issues on the node, while kubectl describe nodereadinessrule tracks rule-wide reconciler failures, without flooding the rule object.

Key updates:

• nodereadinessrule_controller.go: Emit Warning events on the Node for taint errors in evaluateRuleForNode and cleanupTaintsForRule. Emit Warning events on the NodeReadinessRule for reconciler-level failures (finalizer, listing nodes, status patching, dry run, and deletion cleanup).
• node_controller.go: Removed duplicate generic EvaluationError event calls to prevent redundant warning events on the node.
• warning_events_test.go: Added clean unit tests (TestWarningEventsEmittedOnFailures, TestReconcile_ListError, TestReconcile_StatusPatchError) asserting correct targeting scopes and verifying exactly 1 event is emitted on node failures.

Related Issue

Fixes #362

Type of Change

/kind bug

Testing

• Ran make test: All controller unit tests and verification suites pass.
• Updated warning_events_test.go and nodereadinessrule_controller_test.go.

Checklist

• make test passes
• make lint passes

Does this PR introduce a user-facing change?

Emit Warning Kubernetes events on NodeReadinessRule for rule-level failures, and Node objects when taint operations fail.

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[kubernetes-prow]

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This pull-request has been approved by: Sreejesh06
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[kubernetes-prow]

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