Operator does not self-heal collector DaemonSet deletion, causing silent collection outage

# Summary
If the collector DaemonSet `gmp-system/collector` is deleted or never created, the operator logs a warning and exits reconcile successfully without recreating it. Delete events are also filtered out by the controller’s watch predicates. This leaves the cluster without collectors indefinitely and without status surfacing a degraded state.

## Steps to Reproduce
1) Deploy the operator normally so collectors are managed by the operator.
2) Delete the collector DaemonSet: `kubectl -n gmp-system delete ds collector`.
3) Wait for the operator to reconcile.

## Observed Behavior
- Operator logs a warning that the DaemonSet does not exist, returns success, and does not recreate it. See `ensureCollectorDaemonSet` handling of NotFound in [pkg/operator/collection.go#L219-L231](https://github.com/GoogleCloudPlatform/prometheus-engine/blob/main/pkg/operator/collection.go#L219-L231).
- Delete events are not enqueued because the controller watches the DaemonSet with `GenerationChangedPredicate`, which ignores deletes; no reconcile is triggered on deletion (same file around [pkg/operator/collection.go#L106-L113](https://github.com/GoogleCloudPlatform/prometheus-engine/blob/main/pkg/operator/collection.go#L106-L113)).
- Collection remains down permanently; no status condition reflects the loss of collectors.

## Expected Behavior
- The operator should reconcile on DaemonSet deletion and recreate or at least mark degraded, so collectors come back automatically and the condition is visible to users.

## Impact
- Cluster-wide metrics ingestion halts until a human notices and re-applies the DaemonSet. This is a silent data-loss mode: no collectors, no samples, and no operator status signal. In busy clusters, this can wipe out minutes to hours of data and break alerting without clear diagnosis.

## Proposed Fix (high-level)
- Watch DaemonSet delete events (do not filter them out) and enqueue reconcile on delete.
- In `ensureCollectorDaemonSet`, treat NotFound as a reconcile error or recreate the DaemonSet; alternatively set a Degraded condition on the OperatorConfig status so the outage is surfaced.

## Discussion
Please confirm whether this behavior is intentional or a bug

Happy to adjust the understanding or help with a fix/iteration