Add item_id to UserInputTranscribed protobuf

[dishasingh-21]

Fixes #1641
This PR adds item_id to the UserInputTranscribed protobuf message and propagates it through the remote session transport.

This change is a follow-up to livekit-agents#6109, which exposes item_id on UserInputTranscribedEvent so consumers can correlate interim and final transcription updates belonging to the same user utterance. While that change makes item_id available to local consumers, the remote session transport currently drops this information because the protobuf schema only includes transcript and is_final.

Without this update, applications using remote sessions cannot reliably correlate transcription updates, deduplicate interim transcripts, or react exactly once per utterance, leading to inconsistent behavior between local and remote deployments.

Changes

• Add item_id to the UserInputTranscribed protobuf message.
• Forward item_id through the remote session transport.
• Preserve utterance correlation metadata across local and remote session boundaries.

Benefits

Remote session consumers can now:

• Correlate interim and final transcripts belonging to the same utterance.
• Deduplicate transcription updates using a stable identifier.
• React exactly once per utterance.
• Use the same logic across local and remote session deployments.

Related to livekit/agents#6109

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