Description:

When a MigPlan is executed and fails after the CreateRegistries stage, it leaves behind an internal registry Deployment. If the same MigPlan is executed again by re-running cutover (without a rollback), the controller incorrectly attempts to modify the existing, stale Deployment.

This action fails because the spec.selector field of a Kubernetes Deployment is immutable. This causes the entire migration to fail, blocking the user. The controller should be idempotent and handle the presence of resources from a previous failed attempt.

This behavior is a regression as confirmed by customer, as this workflow (re-running a failed migration without manual cleanup) was successful in MTC v1.7 and did not require manual intervention.

## Steps to Reproduce

1. Create and execute a MigPlan by creating a MigMigration object.

1. Allow the plan to proceed until the internal migration registry Deployment is created in the openshift-migration namespace.

1. Induce a failure in the migration after this step, or manually cancel it. Do not perform a rollback.

1. Attempt to re-run the same MigPlan by re-running cutover from UI.

1. Observe the status of the new MigMigration.

## Expected Result

The migration controller should recognize that a registry Deployment from a previous attempt already exists. It should gracefully handle this by deleting and recreating the Deployment, allowing the migration to proceed, which was the behavior in MTC v1.7.

## Actual Result

The MigMigration fails immediately with an error in its status conditions, preventing the migration from starting.

The following error is reported in the MigMigration status:

spec:
  migPlanRef:
    name: gpes-build
    namespace: openshift-migration
status:
  conditions:
  - category: Advisory
    durable: true
    message: 'The migration has failed. See: Errors.'
    reason: CreateRegistries
    status: "True"
    type: Failed
  errors:
  - 'Deployment.apps "registry-8f555f7e-138e-4900-a64a-b5095b8e3813-678j6" is invalid:
    spec.selector: Invalid value: v1.LabelSelector{...}: field is immutable'