feat(health): SiteAuditBacklog metric (count + age + bytes) (#23 M6)

tests/ScadaLink.HealthMonitoring.Tests/SiteAuditBacklogMetricTests.cs

@@ -0,0 +1,73 @@
+using ScadaLink.Commons.Types;
+
+namespace ScadaLink.HealthMonitoring.Tests;
+
+/// <summary>
+/// Bundle E (M6-T6) regression coverage. The site-side audit-log SQLite writer
+/// exposes a backlog snapshot (<c>SiteAuditBacklogSnapshot</c>) via the
+/// <c>ISiteAuditQueue.GetBacklogStatsAsync</c> surface. A periodic
+/// <c>SiteAuditBacklogReporter</c> hosted service polls that snapshot and
+/// pushes it into the collector via <see cref="ISiteHealthCollector.UpdateSiteAuditBacklog"/>
+/// so the next <see cref="ISiteHealthCollector.CollectReport"/> includes it in
+/// the report payload as <c>SiteAuditBacklog</c>. Unlike the
+/// SiteAuditWriteFailures / AuditRedactionFailure interval counters, the
+/// backlog snapshot is not reset on collect — the field carries forward
+/// whatever the most recent refresh pushed in.
+/// </summary>
+public class SiteAuditBacklogMetricTests
+{
+    private readonly SiteHealthCollector _collector = new();
+
+    [Fact]
+    public void Update_Then_CollectReport_IncludesBacklog()
+    {
+        var snapshot = new SiteAuditBacklogSnapshot(
+            PendingCount: 42,
+            OldestPendingUtc: new DateTime(2026, 5, 20, 10, 0, 0, DateTimeKind.Utc),
+            OnDiskBytes: 1234567);
+
+        _collector.UpdateSiteAuditBacklog(snapshot);
+
+        var report = _collector.CollectReport("site-1");
+
+        Assert.Equal(snapshot, report.SiteAuditBacklog);
+    }
+
+    [Fact]
+    public void Report_Payload_Includes_SiteAuditBacklog_AsNullByDefault()
+    {
+        // No refresh has been pushed yet — the report carries null so the
+        // central UI can distinguish "no data yet" from "queue empty".
+        var report = _collector.CollectReport("site-1");
+
+        Assert.Null(report.SiteAuditBacklog);
+    }
+
+    [Fact]
+    public void CollectReport_DoesNotReset_SiteAuditBacklog()
+    {
+        // Backlog snapshot is a point-in-time reading, not a per-interval
+        // counter — successive CollectReport calls before the next
+        // SiteAuditBacklogReporter tick MUST keep returning the same snapshot
+        // so a slow refresh cadence doesn't blank the central dashboard.
+        var snapshot = new SiteAuditBacklogSnapshot(
+            PendingCount: 7,
+            OldestPendingUtc: null,
+            OnDiskBytes: 8192);
+
+        _collector.UpdateSiteAuditBacklog(snapshot);
+
+        var first = _collector.CollectReport("site-1");
+        var second = _collector.CollectReport("site-1");
+
+        Assert.Equal(snapshot, first.SiteAuditBacklog);
+        Assert.Equal(snapshot, second.SiteAuditBacklog);
+    }
+
+    [Fact]
+    public void Update_With_Null_Throws_ArgumentNullException()
+    {
+        Assert.Throws<ArgumentNullException>(
+            () => _collector.UpdateSiteAuditBacklog(null!));
+    }
+}