Make list immutable for app logs query client

Author: xiaomi7732

tools/Azure.Mcp.Tools.ApplicationInsights/src/Services/AppLogsQueryClient.cs

@@ -77,6 +77,6 @@ public class AppLogsQueryClient(LogsQueryClient logsQueryClient) : IAppLogsQuery
             };
         }).ToList(); // Use ToList to force conversion to happen now, not on demand
 
-        return rows;
+        return rows.AsReadOnly();
     }
 }