fix(health-monitoring): resolve HealthMonitoring-013,014,016 — shorter-timeout cadence, options validation, injected TimeProvider; HealthMonitoring-015 left open (cross-module design decision)

tests/ScadaLink.HealthMonitoring.Tests/CentralHealthAggregatorTests.cs

@@ -307,6 +307,36 @@ public class CentralHealthAggregatorTests
         Assert.False(_aggregator.GetSiteState(CentralHealthReportLoop.CentralSiteId)!.IsOnline);
     }
 
+    /// <summary>
+    /// HealthMonitoring-013 regression: the offline-check cadence must be derived
+    /// from the *shorter* of <see cref="HealthMonitoringOptions.OfflineTimeout"/>
+    /// and <see cref="HealthMonitoringOptions.CentralOfflineTimeout"/>, so that if
+    /// an operator configures <c>CentralOfflineTimeout</c> smaller than
+    /// <c>OfflineTimeout</c>, central offline detection is still timely instead of
+    /// being delayed by up to a full <c>OfflineTimeout / 2</c>.
+    /// </summary>
+    [Fact]
+    public void CheckInterval_IsHalfTheShorterTimeout()
+    {
+        // Default: OfflineTimeout (60s) is the shorter of the two.
+        Assert.Equal(
+            TimeSpan.FromSeconds(30),
+            CentralHealthAggregator.ComputeCheckInterval(new HealthMonitoringOptions
+            {
+                OfflineTimeout = TimeSpan.FromSeconds(60),
+                CentralOfflineTimeout = TimeSpan.FromMinutes(3)
+            }));
+
+        // Operator configures CentralOfflineTimeout shorter — cadence must adapt.
+        Assert.Equal(
+            TimeSpan.FromSeconds(10),
+            CentralHealthAggregator.ComputeCheckInterval(new HealthMonitoringOptions
+            {
+                OfflineTimeout = TimeSpan.FromSeconds(60),
+                CentralOfflineTimeout = TimeSpan.FromSeconds(20)
+            }));
+    }
+
     [Fact]
     public void SequenceNumberReset_RejectedUntilExceedsPrevMax()
     {