Expose a specific status.condition when sylva-units HelmRelease is Ready.

The SUR becomes Ready only once the sylva-units-status Kustomisation becomes Ready, until then, we only have the resourcesStatus field to check if the sylva-units HelmRelease is Ready:

  status:
    conditions:
    - lastTransitionTime: "2025-09-16T13:07:41Z"
      message: the SylvaUnitsRelease is not yet ready
      observedGeneration: 3
      reason: ResourceNotReady
      status: "False"
      type: Ready
    observedGeneration: -1
    resourcesStatus:
    - kind: GitRepository
      name: sylva-units-8b2d7f3c
      status: InProgress - reconciliation in progress
    - kind: HelmRelease
      name: sylva-units
      status: Resource is Ready
    - kind: Kustomization
      name: sylva-units-status
      status: InProgress - Kustomization generation is 2, but latest observed generation
        is -1

As it is an important milestone in the global reconciliation, we should add a SylvaUnitsReady condition to indicate the status of sylva-units HelmRelease.

Additionally, we could rely on this condition in tests instead of the Ready condition, and remove the special case for preview here