SolrCloud reconciler blocks deletion when providedConfigMap is missing

## Problem

When a `SolrCloud` CR is deleted and its `providedConfigMap` has already been removed from the cluster (e.g., by Helm during an upgrade or uninstall), the reconciler enters an infinite requeue loop and the SolrCloud CR can never be deleted.

The root cause is in [`controllers/solrcloud_controller.go`](https://github.com/apache/solr-operator/blob/e81458cf9a33f346c69613a7fcb464161f0d525c/controllers/solrcloud_controller.go#L212-L219):

```go
if instance.Spec.CustomSolrKubeOptions.ConfigMapOptions != nil && instance.Spec.CustomSolrKubeOptions.ConfigMapOptions.ProvidedConfigMap != "" {
providedConfigMapName := instance.Spec.CustomSolrKubeOptions.ConfigMapOptions.ProvidedConfigMap
foundConfigMap := &corev1.ConfigMap{}
nn := types.NamespacedName{Name: providedConfigMapName, Namespace: instance.Namespace}
err = r.Get(ctx, nn, foundConfigMap)
if err != nil {
return requeueOrNot, err // if they passed a providedConfigMap name, then it must exist
}
```

The comment says *"it must exist"*, but this assumption doesn't hold during deletion. The `Reconcile()` function has no early exit when `DeletionTimestamp` is set - it runs through ZooKeeper reconciliation, Service creation, node services, headless service, and then hits the ConfigMap lookup **before** it can ever reach the storage finalizer logic at line ~448. If the ConfigMap is gone, the error causes an immediate return and requeue, so the finalizer is never removed and the CR is stuck.

## How to reproduce

1. Create a `SolrCloud` CR with `spec.customSolrKubeOptions.configMapOptions.providedConfigMap` pointing to a ConfigMap
2. Delete the ConfigMap
3. Delete the `SolrCloud` CR
4. Observe: the CR gets a `deletionTimestamp` but is never finalized; the operator logs show a recurring `NotFound` error for the ConfigMap on every reconciliation cycle

This also occurs in Helm-managed deployments: when a Helm upgrade removes SolrCloud workloads from values, Helm deletes the ConfigMap (no longer rendered) while the SolrCloud CR still exists, and the operator cannot complete the CR's deletion.

## Affected versions

Confirmed in the controller code as of v0.9.1 (latest stable). The code path has been unchanged through v0.7.0–v0.9.1.

Start in controllers/solrcloud_controller.go at the providedConfigMap lookup around lines 212–219, then trace Reconcile() through the deletion and storage finalizer logic around line 448. Reproduce the missing-ConfigMap deletion sequence and verify that a SolrCloud with a deletionTimestamp can finish finalization without an infinite NotFound requeue.

Written by the indexing model from the issue text.