fix(helm): Set namespace on templates that does not contain a namespace

[Skaronator]

This PR ensures a namespace is set on Helm-generated resources only if the field is completely missing. This fixes #6058 and fixes #6077 and provides the best of both worlds: Helm charts with pre-defined namespaces remain untouched, while charts lacking a namespace field now function as expected.

---

The namespace is now set in the following priority:

1. Value from helm manifest (for charts already containing a namespace field)
2. Value from helmChart[].namespace (if set)
3. Value from namespace kustomization.yaml (If Set)

We don't modify any Namespaces relevant fields outside the metadata block. Helm Charts should already take care of that. (e.g. spec field for ClusterRoleBinding)

---

Checked the issue in #6058 and it is fixed.

/home/niklas/go/bin/kustomize build --enable-helm | grep -A 10 Deployment
kind: Deployment
metadata:
  labels:
    app.kubernetes.io/instance: mariadb-operator
    app.kubernetes.io/managed-by: Helm
    app.kubernetes.io/name: mariadb-operator
    app.kubernetes.io/version: 25.10.4
    helm.sh/chart: mariadb-operator-25.10.4
  name: mariadb-operator
  namespace: mariadb-system
spec:

Checked the issue #6077 and it is fixed.

/home/niklas/go/bin/kustomize build --enable-helm | grep -C 5 gh24bh7t8g
apiVersion: v1
data:
  secret: Zm9v
kind: Secret
metadata:
  name: test-secret-gh24bh7t8g
  namespace: cluster
type: Opaque
---
apiVersion: v1
kind: Service
--
      serviceAccountName: release-name-test-chart
      volumes:
      - name: test-secret
        secret:
          defaultMode: 292
          secretName: test-secret-gh24bh7t8g
      - configMap:
          defaultMode: 292
          name: test-config-g46hh6k8tf
        name: test-conf
---

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @Skaronator. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[koba1t]

/ok-to-test

[mfredenhagen]

Built from source and tested manually 👍

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: davinkevin, mfredenhagen, Skaronator
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