fix: mitigate reconnect loop

[holzgeist]

This PR mitigates a reconnect loop that happens via following path:

• a reconnect takes the route through restartConnection https://github.com/livekit/client-sdk-flutter/blob/v2.4.6/lib/src/core/engine.dart#L862 (e.g. due to sendSimulateScenario(fullReconnect: true))
• _setUpSignalListeners https://github.com/livekit/client-sdk-flutter/blob/v2.4.6/lib/src/core/engine.dart#L888 sets up a callback that will initiate negotiation under certain circumstances (https://github.com/livekit/client-sdk-flutter/blob/v2.4.6/lib/src/core/engine.dart#L980)
• if connection is fast enough (or it is enforced? I'm not 100% sure about the details), the connection of the publisher is setup (https://github.com/livekit/client-sdk-flutter/blob/v2.4.6/lib/src/core/engine.dart#L455) before we wait for the event here https://github.com/livekit/client-sdk-flutter/blob/v2.4.6/lib/src/core/engine.dart#L902-L905
• The event won't come again, triggering a timeout, a reconnect and the loop start from the beginning

I extracted the code from sendDataPacket into a function that checks if a publisher is connected before attempting to reconnect and reuse it in restartConnection

[cloudwebrtc]

lg!