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)

examples/realtime/realtime-aws-nova-sonic.py

@@ -30,6 +30,7 @@ from pipecat.processors.aggregators.llm_response_universal import (
 from pipecat.runner.types import RunnerArguments
 from pipecat.runner.utils import create_transport
 from pipecat.services.aws.nova_sonic.llm import AWSNovaSonicLLMService
+from pipecat.services.aws.nova_sonic.session_continuation import SessionContinuationParams
 from pipecat.services.llm_service import FunctionCallParams
 from pipecat.transports.base_transport import BaseTransport, TransportParams
 from pipecat.transports.daily.transport import DailyParams
@@ -132,6 +133,16 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
             voice="tiffany",
             system_instruction=system_instruction,
         ),
+        # Session continuation is enabled by default, allowing seamless
+        # conversations longer than the AWS ~8-minute session limit.
+        # The service rotates sessions in the background with no
+        # user-perceptible interruption. You can tune the threshold or
+        # disable it with: session_continuation=SessionContinuationParams(enabled=False)
+        session_continuation=SessionContinuationParams(
+            # When to start preparing the next session (default: 360 = 6 min).
+            # Lower this (e.g. 20) to see a handoff happen quickly during testing.
+            transition_threshold_seconds=360,
+        ),
         # you could choose to pass tools here rather than via context
         # tools=tools
     )