fix(dashboard): generation-tagged feed subscribers survive pump teardown races; drain-timeout observability

src/ZB.MOM.WW.MxGateway.Server/Dashboard/Components/DashboardPageBase.cs

@@ -1,4 +1,5 @@
 using Microsoft.AspNetCore.Components;
+using Microsoft.Extensions.Logging;
 
 namespace ZB.MOM.WW.MxGateway.Server.Dashboard.Components;
 
@@ -31,6 +32,10 @@ public abstract class DashboardPageBase : ComponentBase, IAsyncDisposable
     [Inject]
     protected IDashboardSnapshotFeed SnapshotFeed { get; set; } = null!;
 
+    /// <summary>Logger used to report a snapshot subscription that ended or would not drain.</summary>
+    [Inject]
+    protected ILogger<DashboardPageBase>? Logger { get; set; }
+
     /// <summary>
     /// The most recent gateway metric snapshot. Synchronously seeded from
     /// <see cref="IDashboardSnapshotService.GetSnapshot"/> for the very first
@@ -62,9 +67,21 @@ public abstract class DashboardPageBase : ComponentBase, IAsyncDisposable
                 await _watchTask.WaitAsync(WatchDrainTimeout).ConfigureAwait(false);
             }
         }
+        catch (TimeoutException)
+        {
+            // Accepted limitation: the abandoned loop still holds its feed subscription, so
+            // the feed's idle gate stays open until it does unwind. There is no way to force
+            // a detach — the loop is parked on a dispatcher that is not draining — so the
+            // warning is the operator's only signal that a circuit teardown wedged.
+            Logger?.LogWarning(
+                "Dashboard page {Page} did not release its snapshot subscription within {Timeout}; "
+                + "the shared snapshot feed stays active until it unwinds.",
+                GetType().Name,
+                WatchDrainTimeout);
+        }
         catch
         {
-            // Disposal-time errors (including a drain timeout) are best-effort.
+            // Other disposal-time errors are best-effort.
         }
 
         _watchCancellation.Dispose();
@@ -87,10 +104,15 @@ public abstract class DashboardPageBase : ComponentBase, IAsyncDisposable
         {
             // The page is going away.
         }
-        catch
+        catch (Exception error)
         {
-            // The feed is best-effort: the last rendered snapshot stays on screen and
-            // the snapshot service keeps serving GetSnapshot() for the next page load.
+            // The feed is best-effort: the last rendered snapshot stays on screen and the
+            // snapshot service keeps serving GetSnapshot() for the next page load. Logged
+            // once here, on the way out of the loop — never per snapshot.
+            Logger?.LogWarning(
+                error,
+                "Live snapshot updates ended for dashboard page {Page}; it keeps the last rendered snapshot.",
+                GetType().Name);
         }
     }
 }