When only IAM integration is configured (via -s3.iam.config) without
traditional S3 identities, the isAuthEnabled flag was not being set,
causing the Auth middleware to bypass all authentication checks.
This fix ensures that when SetIAMIntegration is called with a non-nil
integration, isAuthEnabled is set to true, properly enforcing
authentication for all requests.
Added negative authentication tests:
- TestS3AuthenticationDenied: tests rejection of unauthenticated,
invalid, and expired JWT requests
- TestS3IAMOnlyModeRejectsAnonymous: tests that IAM-only mode
properly rejects anonymous requests
Fixes#7724
b0e0c5aaab