[Keda710]

Fixes #293. Checks bucket existence before ListObjects V1 and V2, including max-keys=0. Adds regression tests for missing and existing buckets.

[the-company-company]

🔵 1 low-severity finding

_{Reviewed commit 8029672.}

[the-company-company]

🔵 Low · Handler-level stat adds a second cluster-wide fan-out per list page and leaves the engine bug in place

The missing-bucket bug comes from Engine::list_one_set. It drops every walk_dir error, VolumeNotFound included, and returns Ok(Vec::new()) when quorum fails. So Engine::list says a missing bucket is an empty one. This PR works around that in the handler: every ListObjects page now runs stat_bucket, which calls stat_vol on all backends in the cluster, and then runs the walk.

Two costs follow:

• Every paginated listing makes one extra round trip to every disk.
• Any other Engine::list caller still gets an empty result for a missing bucket.

A smaller fix is to make list_one_set count VolumeNotFound results and return StorageError::BucketNotFound when they reach quorum, the same logic stat_bucket already uses. The only case that still needs an explicit stat_bucket is the max_keys == 0 short-circuit in list_page, and it can do that check before returning the empty page.

[the-company-company]

✅ No issues found

_{Reviewed commit 17bba24.}