## What this PR does
Replaces the plain text input for `storageClass` fields with an
API-backed dropdown listing available StorageClasses from the cluster.
Follows the same pattern as the `instanceType` dropdown for VMInstance.
Affected applications:
- **Top-level `spec.storageClass`**: ClickHouse, Harbor, HTTPCache,
Kubernetes, MariaDB, MongoDB, NATS, OpenBAO, Postgres, Qdrant,
RabbitMQ, Redis, VMDisk
- **Nested `spec.storage.storageClass`**: FoundationDB
- **Nested `spec.kafka.storageClass` / `spec.zookeeper.storageClass`**:
Kafka
### Release note
```release-note
[dashboard] storageClass fields in stateful app forms now render as a
dropdown populated with available StorageClasses from the cluster,
instead of a free-text input.
```
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
## Release Notes
* **New Features**
* Storage class selection dropdowns now available in configuration forms
for multiple database, messaging, and storage services.
* **Tests**
* Added comprehensive test coverage for storage class configuration
handling.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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