fix: diagnostics tab — run ping/traceroute in cell-wireguard, fix wrong method call

The connectivity endpoint was calling routing_manager.test_connectivity()
(no args, internal health check) instead of test_routing_connectivity(target_ip).
Also ping/traceroute aren't installed in the API container; run them via
docker exec cell-wireguard instead.

Updated test_api_endpoints to mock test_routing_connectivity and cover
the new DELETE /firewall/<id> and GET /live-iptables endpoints.

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

api/app.py

@@ -1638,8 +1638,10 @@ def get_live_iptables():
 def test_routing_connectivity():
     """Test routing connectivity."""
     try:
-        data = request.get_json(silent=True)
-        result = routing_manager.test_connectivity(data)
+        data = request.get_json(silent=True) or {}
+        target_ip = data.get('target_ip', '8.8.8.8')
+        via_peer = data.get('via_peer')
+        result = routing_manager.test_routing_connectivity(target_ip, via_peer)
         return jsonify(result)
     except Exception as e:
         logger.error(f"Error testing routing connectivity: {e}")