fix: requeue at dependency interval for missing object refs

[Bisman-Singh]

Part of fluxcd/flux2#5809

When the referenced Source object does not exist, the controller requeued at the retry interval; when a non-optional substituteFrom ConfigMap/Secret does not exist, the failure fell into the generic reconcile-error path, also requeueing at the retry interval. Both are inconsistent with how not ready dependencies, nil source artifacts and missing artifact files are handled, all of which requeue at the --requeue-dependency interval, and they delay bootstrap ordering when the referenced objects are created shortly after the Kustomization (e.g. via a ResourceSet).

This change:

• requeues at --requeue-dependency when the source object is not found;
• tags build errors caused by a missing referenced Kubernetes object (e.g. substituteFrom ConfigMap/Secret) with a sentinel errBuildRefNotFound and requeues them at --requeue-dependency, alongside the existing fetch.ErrFileNotFound handling. The Ready=False condition with BuildFailedReason and the full error message is still recorded.

The check is deliberately scoped to the build step rather than a blanket apierrors.IsNotFound(reconcileErr): fluxcd/pkg/ssa wraps apply errors with %w, so a blanket check would also reroute apply-time NotFound failures (e.g. a namespace that does not exist) away from their error events.

Tests: new TestKustomizationReconciler_RequeueOnMissingRefs asserts the returned RequeueAfter equals the dependency interval for both scenarios; both subtests were verified to fail against the unfixed code (they observed the 10m retry interval).

Signed-off-by: Bisman-Singh bismanmadaan1@gmail.com

[Bisman-Singh]

Flagging a scope question raised on fluxcd/flux2#5809: unlike helm-controller, this controller has no exponential backoff on these paths. Every reconcile failure already returns ctrl.Result{RequeueAfter: obj.GetRetryInterval()}, and the source-not-found branch used obj.GetRetryInterval() before this change too, so this PR moves a user-tunable retryInterval onto the global --requeue-dependency flag rather than fixing a backoff problem. Holding this one for @matheuscscp's call (discussion) — happy to close it or cut it down to just the substituteFrom case. fluxcd/helm-controller#1577 is unaffected by this question.

[Bisman-Singh]

Closing per the discussion on fluxcd/flux2#5809.