fix: Move MongoDB JSON validation to Monitor.validate()

Move databaseQuery JSON validation from mongodb.js check() method
to Monitor.validate() to follow the same pattern as other monitor
types (e.g., ping monitor validations). This ensures validation
happens at the model level before the check is executed.

server/model/monitor.js

@@ -1857,6 +1857,15 @@ class Monitor extends BeanModel {
                 }
             }
         }
+
+        if (this.type === "mongodb" && this.databaseQuery) {
+            // Validate that databaseQuery is valid JSON
+            try {
+                JSON.parse(this.databaseQuery);
+            } catch (error) {
+                throw new Error(`Invalid JSON in database query: ${error.message}`);
+            }
+        }
     }
 
     /**

server/monitor-types/mongodb.js

@@ -12,11 +12,8 @@ class MongodbMonitorType extends MonitorType {
     async check(monitor, heartbeat, _server) {
         let command = { ping: 1 };
         if (monitor.databaseQuery) {
-            try {
-                command = JSON.parse(monitor.databaseQuery);
-            } catch (error) {
-                throw new Error(`Invalid JSON in database query: ${error.message}`);
-            }
+            // databaseQuery is validated in Monitor.validate(), so we can safely parse it here
+            command = JSON.parse(monitor.databaseQuery);
         }
 
         let result = await this.runMongodbCommand(monitor.databaseConnectionString, command);