clients/dotnet: SDK methods for AcknowledgeAlarm + QueryActiveAlarms (PR E.2)

Seventh PR of the alarms-over-gateway epic
(docs/plans/alarms-over-gateway.md). Depends on PR A.1 (proto, merged)
and E.1 (regen, merged).

Hand-written .NET SDK methods on top of the regenerated proto types:

- MxGatewayClient.AcknowledgeAlarmAsync — routes through the existing
  safe-unary retry pipeline (Acks are idempotent at MxAccess), maps
  Unauthenticated/PermissionDenied RpcExceptions to typed
  MxGatewayAuthenticationException / MxGatewayAuthorizationException
  via GrpcMxGatewayClientTransport.MapRpcException.
- MxGatewayClient.QueryActiveAlarmsAsync — server-streaming
  IAsyncEnumerable<ActiveAlarmSnapshot> mirroring the StreamEvents
  pattern.
- IMxGatewayClientTransport extended; GrpcMxGatewayClientTransport
  implements both methods using the regenerated grpc client.
- FakeGatewayTransport extended with capture lists, exception queue,
  and reply / snapshot enqueue helpers.

CLI version-string assertions updated for the GatewayProtocolVersion
2 → 3 bump from A.1.

The CLI alarms verb (subscribe / acknowledge / query-active) is
deferred to a follow-up — keeping this PR focused on the SDK surface
that lmxopcua's GalaxyDriver consumes in PR B.2. The other-language
SDKs (E.3-E.6) layer the same shape on the regen.

Tests:
- 6 new MxGatewayClientAlarmsTests — request shape, cancellation
  honor (linked-token via retry pipeline), Unauthenticated mapping,
  streaming snapshot enumeration, filter prefix passthrough,
  cancellation during enumeration.
- Full client test suite: 57 passed (was 51; 6 new).

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

clients/dotnet/MxGateway.Client.Tests/MxGatewayClientCliTests.cs

@@ -18,7 +18,7 @@ public sealed class MxGatewayClientCliTests
         var exitCode = MxGatewayClientCli.Run(["version"], output, error);
 
         Assert.Equal(0, exitCode);
-        Assert.Contains("gateway-protocol=2", output.ToString());
+        Assert.Contains("gateway-protocol=3", output.ToString());
         Assert.Contains("worker-protocol=1", output.ToString());
         Assert.Equal(string.Empty, error.ToString());
     }
@@ -33,7 +33,7 @@ public sealed class MxGatewayClientCliTests
         int exitCode = await MxGatewayClientCli.RunAsync(["version", "--json"], output, error);
 
         Assert.Equal(0, exitCode);
-        Assert.Contains("\"gatewayProtocolVersion\":2", output.ToString());
+        Assert.Contains("\"gatewayProtocolVersion\":3", output.ToString());
         Assert.Equal(string.Empty, error.ToString());
     }