Document registry_enabled limitation and add tests

[cipheraxat]

Summary

Fixes #433

This PR addresses the confusion around the registry_enabled field in Kubernetes cluster creation. The field is read-only in the API and cannot be set during cluster creation.

Problem

Users expect to be able to pass registry_enabled: True when creating a Kubernetes cluster, but this parameter is ignored because it's a read-only field in the API response.

Solution

This PR:

1. Documents the limitation clearly in the README with a dedicated section explaining the correct approach
2. Adds comprehensive tests demonstrating the proper workflow:
   • Integration test showing create → add_registry → verify → remove_registry flow
   • Mocked test demonstrating the same behavior

The Correct Approach

To enable container registry integration with a Kubernetes cluster:

# 1. Create the cluster (registry_enabled will be False by default)
cluster = client.kubernetes.create_cluster({
    'name': 'my-cluster',
    'region': 'ams3',
    'version': '1.32',
    'node_pools': [...]
})

# 2. Enable registry integration using add_registry
client.kubernetes.add_registry({
    'cluster_uuids': [cluster['kubernetes_cluster']['id']]
})

# 3. Verify registry is now enabled
updated_cluster = client.kubernetes.get_cluster(cluster_id)
assert updated_cluster['kubernetes_cluster']['registry_enabled'] is True

Changes

• Added documentation to README explaining the registry_enabled limitation
• Added test_kubernetes_cluster_registry_integration integration test
• Added test_kubernetes_cluster_with_registry_integration mocked test
• Both tests include references to issue client.kubernetes.create_cluster does not implement registry_enabled as advertised #433

Testing

All existing tests continue to pass, and the new tests verify the correct behavior.