Replaced the unauthenticated SeaweedFS probe (which accepted any HTTP
< 500 as passing) with a signed S3 ListBuckets request using AWS
Signature V4. Credentials are read from the seaweedfs-s3-credentials
K8s secret; a 200 response confirms authentication is working.
Updated tests to cover missing creds, 403 bad-creds, 502 gateway error,
and URLError cases.