Use single OCI layer selector flag

Signed-off-by: Dan Meier <me.daniel.meier@gmail.com>
Assisted-by: Codex/gpt-5

cmd/flux/testdata/oci/export_with_layer_selector.golden

@@ -0,0 +1,14 @@
+---
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: OCIRepository
+metadata:
+  name: podinfo
+  namespace: flux-system
+spec:
+  interval: 10m0s
+  layerSelector:
+    mediaType: application/vnd.cncf.helm.chart.content.v1.tar+gzip
+    operation: copy
+  ref:
+    tag: 6.3.5
+  url: oci://ghcr.io/stefanprodan/manifests/podinfo