chore: handle finalizer errors

[pmalek]

What this PR does / why we need it:

To prevent the following errors:

2025-10-12T19:15:29Z	ERROR	Reconciler error	{"controller": "controlplane", "controllerGroup": "gateway-operator.konghq.com", "controllerKind": "ControlPlane", "ControlPlane": {"name":"gw-pdx4p-gvf9q","namespace":"941cec66-1780-45c6-8181-6c5ef51b1fb0"}, "namespace": "941cec66-1780-45c6-8181-6c5ef51b1fb0", "name": "gw-pdx4p-gvf9q", "reconcileID": "c6d28baa-d1c9-470f-a76c-5b7f8e1d512c", "error": "failed updating ControlPlane: controlplanes.gateway-operator.konghq.com \"gw-pdx4p-gvf9q\" not found"}
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).reconcileHandler
	/home/runner/go/pkg/mod/sigs.k8s.io/[email protected]/pkg/internal/controller/controller.go:353
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).processNextWorkItem
	/home/runner/go/pkg/mod/sigs.k8s.io/[email protected]/pkg/internal/controller/controller.go:300
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func2.1
	/home/runner/go/pkg/mod/sigs.k8s.io/[email protected]/pkg/internal/controller/controller.go:202

this PR makes the error handling a bit more robust by checking if the error from the API server is a not found error and exists the reconciliation without an error when that happens.

Which issue this PR fixes

Fixes #

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect significant changes