test(storage): drive backend/storage coverage 27.5% -> 100% (#18)

The MinIO storage backend only ships StorageBytes (provisioning is API-side),
so this lifts coverage on every branch: New() default vs custom bucket, invalid
endpoint error, StorageBytes empty-prefix guard, BucketExists 5xx error,
BucketExists 404 (NoSuchBucket) "does not exist" branch, ListObjectVersions
iterator success + 5xx error, delete-marker + zero-byte directory-placeholder
skip paths, ListIncompleteUploads success + 5xx error, provider_resource_id
prefix routing, and an empty-listing zero-bytes path.

Tests use httptest.Server S3 emulators that respond to GetBucketLocation
(?location), HEAD BucketExists, ListObjectVersions (?versions), and
ListMultipartUploads (?uploads) — enough surface for the minio-go client
without spinning a real MinIO container.

Co-authored-by: Manas Srivastava <[email protected]>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mastermanas805