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.");
+        }
     }
 }