feat(gateway): forward operator credentials on alarm acknowledgement

Claude-Session: https://claude.ai/code/session_016J6isfcgqy4SB2RK3ZvFvT

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Grpc/MxAccessGatewayServiceTests.cs

@@ -389,6 +389,22 @@ public sealed class MxAccessGatewayServiceTests
         Assert.Equal(StatusCode.InvalidArgument, exception.StatusCode);
     }
 
+    /// <summary>Verifies AcknowledgeAlarm rejects an empty operator_user.</summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task AcknowledgeAlarm_WithMissingOperatorUser_ThrowsInvalidArgument()
+    {
+        MxAccessGatewayService service = CreateService(new FakeSessionManager());
+
+        RpcException exception = await Assert.ThrowsAsync<RpcException>(
+            async () => await service.AcknowledgeAlarm(
+                new AcknowledgeAlarmRequest { AlarmFullReference = "Galaxy!Area.Tag" },
+                new TestServerCallContext()));
+
+        Assert.Equal(StatusCode.InvalidArgument, exception.StatusCode);
+        Assert.Contains("operator_user", exception.Status.Detail, StringComparison.Ordinal);
+    }
+
     /// <summary>Verifies AcknowledgeAlarm delegates a valid request to the alarm service.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]