Osman Ipek f1b16a672a feat(nova-sonic): add proactive session continuation for conversations >8min ...

Nova Sonic sessions have an AWS-imposed ~8-minute time limit. This adds
transparent session continuation that rotates sessions in the background
before the limit is reached, preserving conversation context with no
user-perceptible interruption.

Implementation follows the AWS reference architecture:
- Monitor loop detects when session age exceeds threshold
- On assistant AUDIO contentStart: start buffering user audio, create next
  session (sessionStart + promptStart + system instruction)
- Track SPECULATIVE/FINAL text counts as completion signal
- On completion signal: send conversation history + audioInputStart +
  buffered audio to next session, then promote immediately
- Close old session in background (non-blocking)
- Dead session detection: recreate next session if idle >30s

Key design decisions:
- Session continuation enabled by default (fundamental for long conversations)
- Conversation history tracked in real-time via _sc_conversation_history
  (independent of pipeline context aggregator which updates asynchronously)
- Completion signal check in _handle_content_end_event (after history update)
  to ensure latest text is included in handoff
- Rolling audio buffer (default 3s) captures user audio during transition
- transition_threshold_seconds capped at 420s (7min) for safety margin
- Unified event methods (_send_text_event, _send_client_event, etc.) accept
  optional stream/prompt_name params, eliminating duplicate SC methods

Also adds:
- SessionContinuationParams config (enabled, threshold, buffer, timeout)

2026-04-24 14:55:55 -07:00

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