Provide clear error when no matching ManagedControlPlaneV2 exists

[christophrj]

What would you like to be added?

A clear reconciliation error message when a managed service resource (e.g. Crossplane, Flux) references a ManagedControlPlaneV2 that doesn't exist.

Example

A user applies:

apiVersion: flux.services.openmcp.cloud/v1alpha1
kind: Flux
metadata:
  name: test
  namespace: default
spec:
  version: 2.16.2

If no ManagedControlPlaneV2 named test exists in namespace default, the reconciler should set a clear error condition on the resource explaining the problem.

Why is this needed?

Currently, users get no feedback when creating a managed service instance that references a non-existent ManagedControlPlaneV2. This makes it hard to understand what went wrong.

Acceptance Criteria

• When a managed service CR is reconciled and no matching ManagedControlPlaneV2 exists, set a clear error condition
• The error message should explain the ManagedControlPlane → ServiceProviderAPI naming convention
• Users should be able to understand and fix the problem based on the error message alone

[christophrj]

based on openmcp-project/service-provider-template#27