feat(auditlog): extend ISiteStreamAuditClient with IngestCachedTelemetryAsync (#23 M3)

Add the second site→central RPC seam alongside the existing M2
IngestAuditEventsAsync. The Bundle D proto already lit up
IngestCachedTelemetry (CachedTelemetryBatch / IngestAck) so this commit
just plumbs the client-side abstraction:

* ISiteStreamAuditClient gains IngestCachedTelemetryAsync(batch, ct).
* NoOpSiteStreamAuditClient implements it returning an empty IngestAck
  (same shape as M2 — production gRPC client lands in M6).
* SyncCallEmissionEndToEndTests' DirectActorSiteStreamAuditClient stub
  throws NotSupportedException from the new method so a regression that
  accidentally routes a cached packet through the sync stub fails loudly.
* New NoOpSiteStreamAuditClientTests cover the null-guard + empty-ack
  contract for both batch shapes.

Bundle E task E1.

tests/ScadaLink.AuditLog.Tests/Integration/SyncCallEmissionEndToEndTests.cs

@@ -337,5 +337,17 @@ public class SyncCallEmissionEndToEndTests : TestKit, IClassFixture<MsSqlMigrati
             }
             return ack;
         }
+
+        /// <summary>
+        /// Bundle E E1: the sync-only end-to-end suite does not exercise the
+        /// cached-telemetry path. Throw if it is ever called from these tests
+        /// so a regression that accidentally routes a cached packet through
+        /// the sync stub fails loudly rather than silently no-op'ing.
+        /// </summary>
+        public Task<IngestAck> IngestCachedTelemetryAsync(CachedTelemetryBatch batch, CancellationToken ct)
+        {
+            throw new NotSupportedException(
+                "Sync-call test stub does not implement cached telemetry — use the M3 cached-call client.");
+        }
     }
 }

tests/ScadaLink.AuditLog.Tests/Site/Telemetry/NoOpSiteStreamAuditClientTests.cs

@@ -0,0 +1,58 @@
+using ScadaLink.AuditLog.Site.Telemetry;
+using ScadaLink.Communication.Grpc;
+
+namespace ScadaLink.AuditLog.Tests.Site.Telemetry;
+
+/// <summary>
+/// Bundle E E1 tests for <see cref="NoOpSiteStreamAuditClient"/>. The NoOp
+/// client is the default <see cref="ISiteStreamAuditClient"/> binding until M6
+/// delivers the gRPC-backed implementation; both <c>IngestAuditEventsAsync</c>
+/// (M2) and <c>IngestCachedTelemetryAsync</c> (M3) must return an empty ack
+/// (no rows flipped to Forwarded) without throwing or partially handling the
+/// batch.
+/// </summary>
+public class NoOpSiteStreamAuditClientTests
+{
+    [Fact]
+    public async Task IngestCachedTelemetryAsync_EmptyBatch_ReturnsEmptyAck()
+    {
+        var sut = new NoOpSiteStreamAuditClient();
+        var batch = new CachedTelemetryBatch();
+
+        var ack = await sut.IngestCachedTelemetryAsync(batch, CancellationToken.None);
+
+        Assert.NotNull(ack);
+        Assert.Empty(ack.AcceptedEventIds);
+    }
+
+    [Fact]
+    public async Task IngestCachedTelemetryAsync_PopulatedBatch_ReturnsEmptyAck()
+    {
+        var sut = new NoOpSiteStreamAuditClient();
+        var batch = new CachedTelemetryBatch();
+        batch.Packets.Add(new CachedTelemetryPacket
+        {
+            AuditEvent = new AuditEventDto
+            {
+                EventId = Guid.NewGuid().ToString(),
+                Channel = "ApiOutbound",
+                Kind = "CachedSubmit",
+                Status = "Submitted",
+            },
+        });
+
+        var ack = await sut.IngestCachedTelemetryAsync(batch, CancellationToken.None);
+
+        // No EventIds flipped — NoOp does not forward to anyone.
+        Assert.Empty(ack.AcceptedEventIds);
+    }
+
+    [Fact]
+    public async Task IngestCachedTelemetryAsync_NullBatch_Throws()
+    {
+        var sut = new NoOpSiteStreamAuditClient();
+
+        await Assert.ThrowsAsync<ArgumentNullException>(
+            () => sut.IngestCachedTelemetryAsync(null!, CancellationToken.None));
+    }
+}