Fix flaky testFirmwareUpdateByObject5_Ok: isolate LeshanClient.stop() exception

Leshan's NotificationDataStore.toKey() can throw NPE when the server reference
is null during CoAP observe-relation cleanup on client shutdown (race condition).
This NPE was caught by the outer try-catch in startUpdating(), which prevented
leshanClient.start() from being called, leaving the simulated device stuck in
UPDATING state and causing the awaitility timeout in the OTA integration test.

Fix: wrap leshanClient.stop(false) in its own try-catch so that a Leshan
internal exception during stop does not abort the subsequent client restart.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

application/src/test/java/org/thingsboard/server/transport/lwm2m/client/FwLwM2MDevice.java

@@ -171,7 +171,14 @@ public class FwLwM2MDevice extends BaseInstanceEnabler implements Destroyable {
 
                 if (this.leshanClient != null) {
                     log.info("Stop/reboot LwM2M client {}", this.leshanClient.getEndpoint(identity));
-                    this.leshanClient.stop(false);
+                    try {
+                        this.leshanClient.stop(false);
+                    } catch (Exception stopEx) {
+                        // Leshan may throw NPE during CoAP observe-relation cleanup when the server
+                        // reference is null (race condition in NotificationDataStore.toKey()).
+                        // The client is still considered stopped at this point — proceed with restart.
+                        log.warn("Exception during LwM2M client stop, proceeding with restart: {}", stopEx.getMessage());
+                    }
 
                     log.info("Start after update fw LwM2M client {}", this.leshanClient.getEndpoint(identity));
                     this.leshanClient.start();