Adds check for active simulator before commands

Implements a check to ensure a simulator is active before processing commands.

This prevents errors and unexpected behavior when commands are sent without an active simulator. The check is performed at the beginning of the command parsing, and logs an error if no simulator is active, returning an empty JSON object.

server/simulators.hpp

@@ -52,6 +52,7 @@ class Simulators : public QObject
     void stop();
     void switch_simulator(std::string name);
     std::vector<std::string> list_simulators() const;
+    bool is_not_active() const;
     Simulator_base *invoke_active_simulator();
     json changed_UI_items();