feat: add CEL validation to make placement type immutable for RP

[weng271190436]

Description of your changes

Fixes #311

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✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull Request Overview

This PR refactors validation rules for ClusterResourcePlacement and ResourcePlacement CRDs by separating the immutability checks for the policy field and the placement type into two distinct validation rules. Previously, a single validation rule checked both conditions, which has been split for better clarity and maintainability.

• Separated validation for policy removal and placement type changes into two distinct rules
• Added nested placement type immutability validation on the PlacementPolicy object
• Updated test expectations to match the new error messages
• Added comprehensive integration tests for ResourcePlacement validation scenarios

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
apis/placement/v1beta1/clusterresourceplacement_types.go	Updated kubebuilder validation annotations to split policy removal check from placement type immutability check; added placement type immutability validation on PlacementSpec.Policy field
config/crd/bases/placement.kubernetes-fleet.io_clusterresourceplacements.yaml	Applied generated CRD changes with separate validation rules for policy removal and placement type immutability
config/crd/bases/placement.kubernetes-fleet.io_resourceplacements.yaml	Applied generated CRD changes for ResourcePlacement CRD with the same validation pattern as ClusterResourcePlacement
test/apis/placement/v1beta1/api_validation_integration_test.go	Updated test error message expectations and added new test context for ResourcePlacement validation covering nil policy and placement type change scenarios

[Copilot]

Pull Request Overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

This validation rule will fail when Policy is initially set from nil to a non-nil value because oldSelf will be nil and accessing oldSelf.placementType will cause an error. The rule should check if oldSelf exists first: rule=\"!has(oldSelf) || !(self.placementType != oldSelf.placementType)\"

Suggested change

	// +kubebuilder:validation:XValidation:rule="!(self.placementType != oldSelf.placementType)",message="placement type is immutable"
	// +kubebuilder:validation:XValidation:rule="!has(oldSelf) || !(self.placementType != oldSelf.placementType)",message="placement type is immutable"