feat(health): surface AuditRedactionFailure in central snapshot (#23 M6)

tests/ScadaLink.AuditLog.Tests/Central/CentralAuditRedactionFailureCounterTests.cs

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+using Akka.Actor;
+using Akka.TestKit.Xunit2;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using ScadaLink.AuditLog;
+using ScadaLink.AuditLog.Central;
+using ScadaLink.AuditLog.Payload;
+
+namespace ScadaLink.AuditLog.Tests.Central;
+
+/// <summary>
+/// Bundle E (M6-T9) coverage for the central-side payload-filter redactor
+/// failure bridge. M5 wired the SITE bridge
+/// (<c>HealthMetricsAuditRedactionFailureCounter</c>) that pushes increments
+/// into the site health report; M6 mirrors that with
+/// <see cref="CentralAuditRedactionFailureCounter"/> so the same payload
+/// filter — when it runs on the central writer paths — surfaces failures on
+/// the central <see cref="AuditCentralHealthSnapshot"/>.
+/// </summary>
+public class CentralAuditRedactionFailureCounterTests : TestKit
+{
+    [Fact]
+    public void Increment_Routes_To_Snapshot()
+    {
+        using var tracker = new SiteAuditTelemetryStalledTracker(Sys);
+        var snapshot = new AuditCentralHealthSnapshot(tracker);
+        var counter = new CentralAuditRedactionFailureCounter(snapshot);
+
+        counter.Increment();
+        counter.Increment();
+        counter.Increment();
+
+        Assert.Equal(3, snapshot.AuditRedactionFailure);
+    }
+
+    [Fact]
+    public void Construction_With_Null_Snapshot_Throws()
+    {
+        Assert.Throws<ArgumentNullException>(
+            () => new CentralAuditRedactionFailureCounter(null!));
+    }
+
+    [Fact]
+    public void AddAuditLogCentralMaintenance_Replaces_IAuditRedactionFailureCounter_With_CentralImpl()
+    {
+        // AddAuditLog registers NoOp; AddAuditLogCentralMaintenance is the
+        // override path. The replaced binding MUST resolve to the central
+        // bridge — a site host that wires AddAuditLogHealthMetricsBridge
+        // instead would resolve to the site bridge (covered in
+        // AddAuditLogTests).
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["AuditLog:SiteWriter:DatabasePath"] = ":memory:",
+            })
+            .Build();
+
+        var services = new ServiceCollection();
+        services.AddSingleton<ILoggerFactory, NullLoggerFactory>();
+        services.AddSingleton(typeof(ILogger<>), typeof(NullLogger<>));
+        // The AuditCentralHealthSnapshot ctor takes the stalled tracker
+        // which itself needs an ActorSystem — register a real system
+        // (test-kit's Sys) so the DI graph composes.
+        services.AddSingleton<ActorSystem>(Sys);
+        services.AddAuditLog(config);
+        services.AddAuditLogCentralMaintenance(config);
+        using var provider = services.BuildServiceProvider();
+
+        var counter = provider.GetRequiredService<IAuditRedactionFailureCounter>();
+
+        Assert.IsType<CentralAuditRedactionFailureCounter>(counter);
+    }
+
+    [Fact]
+    public void AddAuditLog_Default_IAuditRedactionFailureCounter_Is_NoOp()
+    {
+        // Sanity check: without AddAuditLogCentralMaintenance the default
+        // remains the NoOp from M5 — the central bridge only takes effect
+        // when the central-only registration runs.
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["AuditLog:SiteWriter:DatabasePath"] = ":memory:",
+            })
+            .Build();
+
+        var services = new ServiceCollection();
+        services.AddSingleton<ILoggerFactory, NullLoggerFactory>();
+        services.AddSingleton(typeof(ILogger<>), typeof(NullLogger<>));
+        services.AddAuditLog(config);
+        using var provider = services.BuildServiceProvider();
+
+        var counter = provider.GetRequiredService<IAuditRedactionFailureCounter>();
+
+        Assert.IsType<NoOpAuditRedactionFailureCounter>(counter);
+    }
+}