Participant is not unlinked during disconnection, so human input is out of sync

[brightsparc]

When a participant disconnects from an active agent the human input stays connected with the old participant, and so when a participant reconnects, the transcription and sst, vad and transcript forwarder are not connected.

The fix is to add a method to unlink_participant which requires moving a number of nested methods to class level for unlink:

    def _unlink_participant(self) -> None:
        """Clean up participant-related resources"""
        if self._human_input is None:
            return

        # Remove all event listeners using class methods
        self._human_input.off("start_of_speech", self._on_start_of_speech)
        self._human_input.off("vad_inference_done", self._on_vad_inference_done)
        self._human_input.off("end_of_speech", self._on_end_of_speech)
        self._human_input.off("interim_transcript", self._on_interim_transcript)
        self._human_input.off("final_transcript", self._on_final_transcript)
        self._human_input = None

Steps to reproduce

1. Start room with participant
2. Have agent join
3. Leave room but have agent stay connected
4. Rejoin room
5. Have candidate speak and notice that transcription is not being forwarded.