clients/java: SDK methods for AcknowledgeAlarm + QueryActiveAlarms (PR E.5)

Tenth PR of the alarms-over-gateway epic
(docs/plans/alarms-over-gateway.md). Mirrors PR E.2's .NET surface
on the Java SDK. Depends on PR E.1 (regen, merged).

- MxGatewayClient.acknowledgeAlarm — blocking unary call, validates
  protocol status via the existing MxGatewayErrors helper. Wraps
  RuntimeException through MxGatewayErrors.fromGrpc for typed
  failure mapping.
- MxGatewayClient.acknowledgeAlarmAsync — CompletableFuture variant
  using the future stub.
- MxGatewayClient.queryActiveAlarms — async server-streaming RPC
  observed via a new MxGatewayActiveAlarmsSubscription handle
  (parallel to MxGatewayEventSubscription; the existing
  subscription class is hard-typed to MxEvent so a parallel type
  was simpler than retrofitting generics).
- MxGatewayClientVersion bumps GATEWAY_PROTOCOL_VERSION 2 → 3 to
  match the .NET contract; CLI version-string assertions updated
  to match.

Java SDK build green via Gradle 9.4.1 (mxgateway-client + mxgateway-cli).
17 tasks, all tests passing.

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

clients/java/mxgateway-cli/src/test/java/com/dohertylan/mxgateway/cli/MxGatewayCliTests.java

@@ -32,7 +32,7 @@ final class MxGatewayCliTests {
         assertEquals(0, run.exitCode());
         assertEquals("", run.errors());
         assertTrue(run.output().contains("mxgateway-java 0.1.0"));
-        assertTrue(run.output().contains("gatewayProtocolVersion=1"));
+        assertTrue(run.output().contains("gatewayProtocolVersion=3"));
         assertTrue(run.output().contains("workerProtocolVersion=1"));
     }
 
@@ -42,7 +42,7 @@ final class MxGatewayCliTests {
 
         assertEquals(0, run.exitCode());
         assertTrue(run.output().contains("\"clientVersion\":\"0.1.0\""));
-        assertTrue(run.output().contains("\"gatewayProtocolVersion\":1"));
+        assertTrue(run.output().contains("\"gatewayProtocolVersion\":3"));
     }
 
     @Test