[opensearch-k8s-operator] Dashboards Deployment selector built from all spec.dashboards.labels

## Observed symptom [BUG] Dashboards Deployment selector built from all `spec.dashboards.labels` ### Description The operator copies every entry of `spec.dashboards.labels` into the `spec.selector.matchLabels` of the generated OpenSearch Dashboards Deployment. Since a Deployment's selector is immutable, any later change to `spec.dashboards.labels` (adding, removing or changing a label) makes the reconcile fail. This is a practical problem for Helm-managed CRs: charts commonly add labels such as `helm.sh/chart: <chart>-<version>`, whose value changes on every chart upgrade. ### How can one reproduce the bug? 1. Operator v3 (earlier versions as well), create an `OpenSearchCluster` with: ```yaml spec: dashboards: enable: true labels: app.kubernetes.io/in ## Literal error text ``` Dashboards Deployment selector from all labels ``` ## Environment GitHub opensearch-project/opensearch-k8s-operator #1642 ## Trigger / context ### Description The operator copies every entry of `spec.dashboards.labels` into the `spec.selector.matchLabels` of the generated OpenSearch Dashboards Deployment. Since a Deployment's selector is immutable, any later change to `spec.dashboards.labels` (adding, removing or changing a label) makes the reconcile fail. This is a practical problem for Helm-managed CRs: charts commonly add labels such as `helm.sh/chart: <chart>-<version>`, whose value changes on every chart upgrade. ### How can one reproduce the bug? 1. Operator v3 (earlier versions as well), create an `OpenSearchCluster` with: ```yaml spec: dashboards: enable: true labels: app.kubernetes.io/instance: opensearch-cluster app.kubernetes.io/managed-by: Helm app.kubernetes.io/name: opensearch-cluster helm.sh/chart: opensearch-cluster-4.0.0 ``` 2. Inspect the generated Deployment. All labels above end up in the selector: ```yaml spec: selector: match ## Source evidence - https://github.com/opensearch-project/opensearch-k8s-operator/issues/1642 (2026-10-02T08:16:26Z) — #1642 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:49:02.645589Z - scout_id: Cfd0a7176e0 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.