refactor(uns): clarify service lifetime doc + defensive vtag-count null filter (review)

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/UnsTreeService.cs

@@ -9,9 +9,9 @@ namespace ZB.MOM.WW.OtOpcUa.AdminUI.Uns;
 /// rows to the pure <see cref="UnsTreeAssembly.Build"/> to nest into the browse tree.
 /// </summary>
 /// <remarks>
-/// A new context is created per call via the pooled factory — the same pattern every
-/// AdminUI page uses — so the service is safe to register as a scoped singleton and call
-/// concurrently from independent Blazor circuits.
+/// Each call creates and disposes its own context via the pooled factory — the same pattern
+/// every AdminUI page uses — so the service is safe to register as Scoped and used per
+/// Blazor circuit.
 /// </remarks>
 public sealed class UnsTreeService(IDbContextFactory<OtOpcUaConfigDbContext> dbFactory) : IUnsTreeService
 {
@@ -55,11 +55,12 @@ public sealed class UnsTreeService(IDbContextFactory<OtOpcUaConfigDbContext> dbF
                 .ToListAsync(ct))
             .ToDictionary(x => x.EquipmentId, x => x.Count, StringComparer.Ordinal);
 
-        // Per-equipment virtual-tag counts (EquipmentId is always set on virtual tags).
+        // Per-equipment virtual-tag counts (virtual tags with no equipment are excluded).
         var vtagCounts = (await db.VirtualTags
                 .AsNoTracking()
+                .Where(v => v.EquipmentId != null)
                 .GroupBy(v => v.EquipmentId)
-                .Select(g => new { EquipmentId = g.Key, Count = g.Count() })
+                .Select(g => new { EquipmentId = g.Key!, Count = g.Count() })
                 .ToListAsync(ct))
             .ToDictionary(x => x.EquipmentId, x => x.Count, StringComparer.Ordinal);