s3: enable auth when IAM integration is configured (#7726)

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

weed/s3api/auth_credentials.go

@@ -771,6 +771,11 @@ func (iam *IdentityAccessManagement) SetIAMIntegration(integration *S3IAMIntegra
 	iam.m.Lock()
 	defer iam.m.Unlock()
 	iam.iamIntegration = integration
+	// When IAM integration is configured, authentication must be enabled
+	// to ensure requests go through proper auth checks
+	if integration != nil {
+		iam.isAuthEnabled = true
+	}
 }
 
 // authenticateJWTWithIAM authenticates JWT tokens using the IAM integration