test(historian-gateway): cover AlarmHistorianOptions.Validate MaxAttempts<=0 warning (FU-4)

The MaxAttempts<=0 warning branch in AlarmHistorianOptions.Validate() was the
only one without a test (the sibling DrainIntervalSeconds/Capacity/
DeadLetterRetentionDays warnings are covered). Add the matching case. Closes FU-4.

Claude-Session: https://claude.ai/code/session_012SDSQ3AcaXqPcBtDESBRii

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Historian/AlarmHistorianRegistrationTests.cs

@@ -152,6 +152,13 @@ public sealed class AlarmHistorianRegistrationTests
         opts.Validate().ShouldContain(w => w.Contains("DeadLetterRetentionDays"));
     }
 
+    [Fact]
+    public void Validate_warns_on_non_positive_max_attempts()
+    {
+        var opts = new AlarmHistorianOptions { Enabled = true, DatabasePath = "/abs/h.db", MaxAttempts = 0 };
+        opts.Validate().ShouldContain(w => w.Contains("MaxAttempts"));
+    }
+
     [Fact]
     public void Validate_accumulates_multiple_warnings()
     {