feat(aws): expose Nova Sonic session ID on the realtime session

[guiruggiero]

What

Stores Nova Sonic's sessionId on RealtimeSession, exposes it as a session_id property, and logs it once per stream at INFO level.

Purely additive — no change to the existing event flow.

Why

I'm the PM for Amazon Nova Sonic. When a customer running Sonic through LiveKit files a support ticket with us, the first thing we ask for is the session ID, and today there's no easy way for them to get it — which blocks debugging on our side.

Nova Sonic stamps sessionId on every server event and it's the canonical identifier for tracing a session on the service side, but the plugin never stores it. The only place it currently surfaces is inside the DEBUG-level log_event_data() call on completionStart, so users have to run the worker at --log-level debug and scrape the raw JSON out of the lk.pii.event_data field. The log_event_data() call in _handle_usage_event is commented out, so the earliest occurrence isn't logged at all.

Details

Why usageEvent rather than completionStart: usageEvent is the earliest server event carrying sessionId. I confirmed this against the live Bedrock API with a session that sends only the init sequence (sessionStart → promptStart → SYSTEM → SYSTEM_SPEECH) and no user input at all: four usageEvents arrived, each carrying sessionId, before any completionStart. Capturing it here means the ID is available before the user has said anything, so it survives a session that fails early.

I also traced a full session (100+ events — usageEvent, completionStart, contentStart, textOutput, audioOutput, contentEnd, completionEnd) and every one carried an identical sessionId, so reading it off any single event is equivalent. usageEvent is simply the first opportunity.

Why log on change rather than once: _session_recycle_timer and initialize_streams(is_restart=True) open a new Bedrock stream — ahead of the service's session-duration limit and on credential refresh — and each new stream is assigned a new sessionId. So a single RealtimeSession legitimately spans several IDs. The != guard emits one line per ID instead of one per usageEvent.

Naming: session_id -> str | None follows the existing convention in the codebase — the OpenAI realtime plugin (gpt_live_model.py), the AssemblyAI and Speechmatics STT plugins, and inference/avatar.py all expose the same shape.

Checks

• ruff check and ruff format --check pass, verified against the 0.15.22 pinned in uv.lock
• The new property is documented, per CONTRIBUTING.md
• No CHANGELOG.md or manifest edits, per CONTRIBUTING.md