refactor(room-io): use protocol TimedString for jsonFormat output

[toubatbrian]

Replace the inline Record<string, unknown> + JSON.stringify path in ParticipantTranscriptionOutput.encodeJsonChunk with the @livekit/protocol TimedString proto class (added in livekit/protocol#1502). agents-js already depends on @livekit/protocol, so use it directly to mirror the Python implementation's agent_pb.TimedString + MessageToDict serialization. Bumps @livekit/protocol to ^1.45.6 to pick up the TimedString message.

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