feat(upnp): add integration tests

[ackintosh]

Description

• Add integration tests for the UPnP behaviour using the mock-igd crate
• Introduce test_mode to enable testing without real UPnP infrastructure

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[jxs]

Hi Akihito! Thanks for this, but changing the ci workflow and the Behavior with fields just for test seem like a code smell to me, can we avoid this?
thanks!