* s3api: make RoleArn optional in AssumeRole
* s3api: address PR feedback for optional RoleArn
* iam: add configurable default role for AssumeRole
* S3 STS: Use caller identity when RoleArn is missing
- Fallback to PrincipalArn/Context in AssumeRole if RoleArn is empty
- Handle User ARNs in prepareSTSCredentials
- Fix PrincipalArn generation for env var credentials
* Test: Add unit test for AssumeRole caller identity fallback
* fix(s3api): propagate admin permissions to assumed role session when using caller identity fallback
* STS: Fix is_admin propagation and optimize IAM policy evaluation for assumed roles
- Restore is_admin propagation via JWT req_ctx
- Optimize IsActionAllowed to skip role lookups for admin sessions
- Ensure session policies are still applied for downscoping
- Remove debug logging
- Fix syntax errors in cleanup
* fix(iam): resolve STS policy bypass for admin sessions
- Fixed IsActionAllowed in iam_manager.go to correctly identify and validate internal STS tokens, ensuring session policies are enforced.
- Refactored VerifyActionPermission in auth_credentials.go to properly handle session tokens and avoid legacy authorization short-circuits.
- Added debug logging for better tracing of policy evaluation and session validation.
