test(site-runtime): flush the tracking-store queue before the boundary-timestamp test's direct UPDATE

The single-writer queue (d91135f9) made RecordEnqueueAsync return on enqueue, so the
test's verifier-connection UPDATE raced the pump and touched zero rows whenever the pump
lost. Every other verifier-connection test already flushed; this one was missed.

Claude-Session: https://claude.ai/code/session_01NcMG6xXdsJbmD2wYAgkK29

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Tracking/OperationTrackingStoreTests.cs

@@ -872,6 +872,11 @@ public class OperationTrackingStoreTests : IDisposable
             kind: nameof(AuditKind.ApiCallCached), targetSummary: "ERP.B",
             sourceInstanceId: "I", sourceScript: "S", sourceNode: "node-a");
 
+        // Writes are queued (single-writer pump, 2026-09-06): the verifier connection
+        // below bypasses the store, so drain the queue first or the UPDATE races the
+        // INSERTs and touches zero rows.
+        await store.FlushAsync();
+
         // Force both rows to share one exact UpdatedAtUtc instant, formatted the same
         // way the read normalises the cursor ('Z'-suffixed round-trip "o").
         var boundary = DateTime.UtcNow;