###What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Consolidates scattered hardcoded strings and inline GVK/GVR definitions into canonical constants, eliminating duplicated magic strings across the e2e codebase.

GVK consolidation:

• Created e2e/grove/gvk/gvk.go — canonical GVK constants for all Grove CRDs (PodCliqueSet, PodClique, PodCliqueScalingGroup, PodGang) and third-party CRDs (ComputeDomain)
• Created e2e/setup/gvk.go — resourceType definitions and groveManagedResourceTypes moved from shared_cluster.go, with kindToResource and newResourceType helpers
• Created e2e/tests/gvk.go — test-local GVK constants for cert-manager and apiextensions CRDs
• Replaced all inline schema.GroupVersionKind{...} literals in workload.go, diagnostics/collector.go, shared_cluster.go, resource_sharing_test.go, cert_management_test.go, crd_installer_test.go with imports from the canonical packages

Label constant consolidation:

• Replaced all hardcoded "app.kubernetes.io/part-of" with common.LabelPartOfKey
• Replaced all hardcoded "app.kubernetes.io/name" with common.LabelAppNameKey
• Replaced all hardcoded "app.kubernetes.io/component" with common.LabelComponentKey
• Replaced all hardcoded "app.kubernetes.io/managed-by" with common.LabelManagedByKey
• Replaced all hardcoded "grove-operator" label values with common.LabelManagedByValue or setup.OperatorDeploymentName
• Replaced all hardcoded "grove-system" with setup.OperatorNamespace

Other improvements:

• WaitForPodCliqueSetDeletion standalone function added
• WorkloadManager.WaitForPCSG, WaitForPodClique, DeletePCSAndWait now delegate to standalone functions
• reapplyNodeLabels uses targeted JSON merge patch instead of full-object MergeFrom

Which issue(s) this PR fixes:

Fixes #520

Special notes for your reviewer:

• gvk.go files follow a convention: the package that operates on the CRD owns the GVK definition. grove/gvk/ for Grove + third-party CRDs, setup/gvk.go for cleanup resource types, tests/gvk.go for test-only CRDs.
• The kindToResource helper assumes the standard K8s convention (strings.ToLower(kind) + "s"), which holds for all CRDs in this codebase.
• Kubernetes core resource GVKs (Service, Secret, Role, etc.) are defined locally in setup/gvk.go since they're only used for cleanup operations.
• e2e/utils/pprof/pprof.go still has "grove-system" in a comment — not a code reference, left as-is.

Does this PR introduce a API change?

 NONE

Additional documentation e.g., enhancement proposals, usage docs, etc.:

NONE