fix(centralui): short negative TTL after a faulted pending-updates flight (review 2026-10-21 04/C4)

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Services/PendingUpdatesCacheTests.cs

@@ -115,10 +115,10 @@ public class PendingUpdatesCacheTests : IDisposable
         await _evaluator.Received(1).EvaluateAsync(Arg.Any<CancellationToken>());
     }
 
-    /// <summary>Verifies that a failed evaluation surfaces to the caller and is not memoized.</summary>
+    /// <summary>Verifies that a failed evaluation surfaces to the caller and is remembered only for the short back-off (review 2026-10-21 04/C4).</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
-    public async Task FailedEvaluation_SurfacesToTheCaller_AndIsNotMemoized()
+    public async Task FailedEvaluation_SurfacesToTheCaller_AndIsBackedOffNotRetriedPerPoll()
     {
         _evaluator.EvaluateAsync(Arg.Any<CancellationToken>())
             .Returns(
@@ -128,10 +128,12 @@ public class PendingUpdatesCacheTests : IDisposable
 
         await Assert.ThrowsAsync<InvalidOperationException>(() => sut.GetAsync());
 
-        // Same instant, well inside the TTL — a cached fault would starve every dashboard
-        // for the rest of the window, so the next caller must start a new evaluation.
-        var recovered = await sut.GetAsync();
+        // Inside the negative TTL a poll is served the same fault with no new flatten.
+        await Assert.ThrowsAsync<InvalidOperationException>(() => sut.GetAsync());
+        await _evaluator.Received(1).EvaluateAsync(Arg.Any<CancellationToken>());
 
+        // An operator Re-check bypasses the back-off.
+        var recovered = await sut.GetAsync(forceRefresh: true);
         Assert.NotNull(recovered);
         await _evaluator.Received(2).EvaluateAsync(Arg.Any<CancellationToken>());
     }