fix: ignore duplicate native signaling events

[mertushka]

Summary

Fix late duplicate native signalingstatechange notifications when the JavaScript facade has already exposed the same description-derived signaling state.

This keeps repeated remote offers from reporting an extra signalingstatechange after the first have-remote-offer transition.

Impact

• Adds focused regression coverage for a late native duplicate after setRemoteDescription(offer).
• No public API, native addon, WPT selection, or documentation changes.