fix: aws connection is failing in OpenRAG version 0 4 0 dev5

[ricofurtado]

• Implement verify_s3_credentials to validate AWS S3 credentials.
• Update S3Connector.authenticate to use the new verification method.
• Add unit tests for verify_s3_credentials covering various scenarios.

There is a behaviour question(maybe product can anwer it):

• If the credentials are valid but don’t have s3:ListAllMyBuckets, “Test connection” should still succeed?
  In this case, the UI should not block saving just because bucket listing failed.

• If bucket listing is unavailable, do we need to explain why in plain terms? E.g. “Credentials are valid, but this IAM user can’t list all buckets. Enter/select bucket names manually.”

-After saving, the connection should only be treated as usable if it has either:
ListBuckets permission, so auto-discovery can work, or
explicit bucket_names, so the connector can operate without bucket discovery.

So the practical fix is to split today’s combined step into two states:

• validate credentials
• try to discover buckets
• Bucket discovery should be best-effort, not a prerequisite for saving. If discovery fails with AccessDenied, we should still save the connection and guide the user into a manual bucket-name path instead of surfacing a generic failure.

[edwinjosechittilappilly]

Rico can you check with QA team . that S3 and COS would have been missing since we disabled it for OpenSource and works only in IBM cloud deployments.