feat: Improve service manager testing and error handling

- Add comprehensive testing instructions to README.
- Improve error handling in examples to prevent crashes.
- Enhance launchctl error handling for production safety.
- Improve zinit error handling for production safety.
- Remove obsolete plan_to_fix.md file.
- Update Rhai integration tests for improved robustness.
- Improve service manager creation on Linux with systemd fallback.

service_manager/README.md

@@ -129,6 +129,34 @@ herodo examples/service_manager/basic_usage.rhai
 
 See `examples/service_manager/README.md` for detailed documentation.
 
+## Testing
+
+Run the test suite:
+
+```bash
+cargo test -p sal-service-manager
+```
+
+For Rhai integration tests:
+
+```bash
+cargo test -p sal-service-manager --features rhai
+```
+
+### Testing with Herodo
+
+To test the service manager with real Rhai scripts using herodo, first build herodo:
+
+```bash
+./build_herodo.sh
+```
+
+Then run Rhai scripts that use the service manager:
+
+```bash
+herodo your_service_script.rhai
+```
+
 ## Prerequisites
 
 ### Linux (zinit)