fix(test): restore #[serial] markers the fallback runner still needs (#6767)

crates/scanner/src/lib.rs

@@ -951,8 +951,10 @@ impl ScannerConfigObjectDelete for SetDisks {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use serial_test::serial;
 
     #[tokio::test]
+    #[serial]
     async fn runtime_tier_names_serves_cached_arc_within_ttl() {
         reset_tier_name_cache_for_test();
         // The tier config manager is unconfigured in unit tests, so the
@@ -1028,6 +1030,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn foreground_read_guard_tracks_stream_lifetime() {
         reset_foreground_read_activity_for_test();
         assert_eq!(current_foreground_read_activity(), 0);
@@ -1041,6 +1044,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn foreground_read_activity_keeps_larger_signal() {
         reset_foreground_read_activity_for_test();
         let _guard = ForegroundReadGuard::new();
@@ -1053,6 +1057,7 @@ mod tests {
     }
 
     #[test]
+    #[serial]
     fn scanner_runtime_guard_tracks_runtime_lifetime() {
         reset_scanner_runtime_instances_for_test();
         assert!(!scanner_runtime_initialized());