gateway: alarm-RPC dispatcher seam (PRs A.6 + A.7)

Replaces the inline diagnostic strings in PR A.3's AcknowledgeAlarm
+ QueryActiveAlarms handlers with an IAlarmRpcDispatcher seam.

- IAlarmRpcDispatcher (new) — gateway-side abstraction over the
  worker-RPC path that fronts AlarmClient.AlarmAckByGUID and the
  active-alarm walk. AcknowledgeAsync returns the
  AcknowledgeAlarmReply directly; QueryActiveAlarmsAsync yields an
  IAsyncEnumerable<ActiveAlarmSnapshot>.
- NotWiredAlarmRpcDispatcher (new, default impl) — returns
  PROTOCOL_STATUS_OK with a structured worker-pending diagnostic
  on Acknowledge, yields an empty stream on QueryActiveAlarms.
  Same observable shape as PR A.3, but the integration seam is
  now in code instead of hardcoded inside the handler.
- MxAccessGatewayService — handlers delegate to the dispatcher.
  Constructor accepts an optional IAlarmRpcDispatcher (default
  NotWiredAlarmRpcDispatcher); a future WorkerAlarmRpcDispatcher
  registration in DI swaps in the live worker-IPC routing without
  changing the public RPC surface.
- 2 new dispatcher tests pin the not-wired contract; 279 → 281
  total tests, all green.

Worker-side dispatch (translating Acknowledge / QueryActiveAlarms
to the IPC method that calls IMxAccessAlarmConsumer from PR A.5)
is the dev-rig follow-up — it depends on validating the AVEVA
GetAlarmChangesCompleted event subscription against a live alarm
provider before pinning a wire format.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/MxGateway.Server/Sessions/IAlarmRpcDispatcher.cs

@@ -0,0 +1,41 @@
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using MxGateway.Contracts.Proto;
+
+namespace MxGateway.Server.Sessions;
+
+/// <summary>
+///     PR A.6 / A.7 — gateway-side dispatcher for the alarm-RPC surface.
+///     Bridges the public <c>AcknowledgeAlarm</c> + <c>QueryActiveAlarms</c>
+///     gRPC handlers to the worker process that hosts
+///     <c>IMxAccessAlarmConsumer</c>.
+/// </summary>
+/// <remarks>
+///     <para>
+///         Production implementations live in <c>WorkerAlarmRpcDispatcher</c>
+///         (this PR ships a not-yet-wired default that returns a clear
+///         worker-pending diagnostic) and route through the existing
+///         worker-pipe IPC. Tests inject a fake to exercise the gateway
+///         handler shape without spinning up a worker process.
+///     </para>
+///     <para>
+///         The dispatcher is session-scoped: every call resolves the
+///         session and forwards to that session's worker. The handler
+///         constructs the <see cref="AcknowledgeAlarmReply"/> /
+///         <see cref="ActiveAlarmSnapshot"/> stream from the dispatcher's
+///         output without further translation.
+///     </para>
+/// </remarks>
+public interface IAlarmRpcDispatcher
+{
+    /// <summary>Forward an Acknowledge to the worker that owns the session.</summary>
+    Task<AcknowledgeAlarmReply> AcknowledgeAsync(
+        AcknowledgeAlarmRequest request,
+        CancellationToken cancellationToken);
+
+    /// <summary>Walk active alarms on the worker that owns the session.</summary>
+    IAsyncEnumerable<ActiveAlarmSnapshot> QueryActiveAlarmsAsync(
+        QueryActiveAlarmsRequest request,
+        CancellationToken cancellationToken);
+}