Add validation logic to OpenstackCreds webhook #2347

Summary

The OpenstackCredsCustomValidator webhook scaffold was added in # but the ValidateCreate, ValidateUpdate, and ValidateDelete methods are currently no-ops.

Work Required

• Implement validation logic in k8s/migration/api/v1alpha1/openstackcreds_webhook.go:
  • ValidateCreate: validate required fields (auth URL, username, password, tenant, domain, region) are non-empty
  • ValidateUpdate: validate same fields; optionally prevent immutable field changes (e.g. auth URL)
  • ValidateDelete: optionally block deletion if referenced by an active MigrationPlan
• Expand openstackcreds_webhook_test.go with table-driven tests covering accept/reject cases for each operation
• Remove the TODO(user) placeholders in the test file

Acceptance Criteria

• Webhook rejects OpenstackCreds with missing required fields at admission time
• Unit tests cover at minimum: missing auth URL, missing username, valid object, update with valid object