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

src/ScadaLink.HealthMonitoring/CentralHealthAggregator.cs

@@ -191,9 +191,10 @@ public class CentralHealthAggregator : BackgroundService, ICentralHealthAggregat
             "Central health aggregator started, offline timeout {Timeout}s (central {CentralTimeout}s)",
             _options.OfflineTimeout.TotalSeconds, _options.CentralOfflineTimeout.TotalSeconds);
 
-        // Check at half the (shorter) offline timeout interval for timely detection
-        var checkInterval = TimeSpan.FromMilliseconds(_options.OfflineTimeout.TotalMilliseconds / 2);
-        using var timer = new PeriodicTimer(checkInterval);
+        // Check at half the shorter of the two offline timeouts so detection is
+        // timely for whichever site class (real or "central") has the tighter
+        // window — see ComputeCheckInterval.
+        using var timer = new PeriodicTimer(ComputeCheckInterval(_options));
 
         while (await timer.WaitForNextTickAsync(stoppingToken).ConfigureAwait(false))
         {
@@ -201,6 +202,24 @@ public class CentralHealthAggregator : BackgroundService, ICentralHealthAggregat
         }
     }
 
+    /// <summary>
+    /// Computes the offline-check timer cadence: half of the <em>shorter</em> of
+    /// <see cref="HealthMonitoringOptions.OfflineTimeout"/> and
+    /// <see cref="HealthMonitoringOptions.CentralOfflineTimeout"/>. Deriving it
+    /// from the shorter timeout guarantees that whichever site class has the
+    /// tighter window is still polled at least twice within it — so if an
+    /// operator configures <c>CentralOfflineTimeout</c> smaller than
+    /// <c>OfflineTimeout</c>, central offline detection is not delayed by up to a
+    /// full <c>OfflineTimeout / 2</c>.
+    /// </summary>
+    internal static TimeSpan ComputeCheckInterval(HealthMonitoringOptions options)
+    {
+        var shorter = options.OfflineTimeout < options.CentralOfflineTimeout
+            ? options.OfflineTimeout
+            : options.CentralOfflineTimeout;
+        return TimeSpan.FromMilliseconds(shorter.TotalMilliseconds / 2);
+    }
+
     internal void CheckForOfflineSites()
     {
         var now = _timeProvider.GetUtcNow();