Fix issue #7946 where SeaweedFS only recognized legacy IAM role ARN format
(arn:aws:iam::role/RoleName) but not the standard AWS format with account ID
(arn:aws:iam::ACCOUNT:role/RoleName). This was breaking EKS pod identity
integration which expects the standard format.
Changes:
- Update ExtractRoleNameFromArn() to handle both formats by searching for
'role/' marker instead of matching a fixed prefix
- Update ExtractRoleNameFromPrincipal() to clearly document both STS and IAM
formats it supports
- Simplify role ARN validation in validateRoleAssumptionForWebIdentity() and
validateRoleAssumptionForCredentials() to use the extraction function
The fix maintains backward compatibility with legacy format while adding
support for standard AWS format with account ID.
Fixes: https://github.com/seaweedfs/seaweedfs/issues/7946