feat(aws-stt): add keep-alive and robust retry logic

[itskyf]

This PR refactors the Amazon Transcribe STT plugin to improve session stability, and billing efficiency.

Changes & Improvements

• Audio Keep-Alive:
  • Old behavior: Let the stream time out after 15s of silence, incurring reconnection latency and repeated minimum 15-second billing charges.
  • New behavior: Sends silent PCM frames every 10s to maintain a long-lived session, ensuring instant response and better cost efficiency.
• Replaced asyncio.gather with asyncio.TaskGroup: if either the send or receive loop fails for safer cleanup (please check the below notes).
• Moved the audio iterator outside the retry loop. If a network error occurs, the plugin reconnects without "forgetting" the buffered audio.

Notes

• I removed runtime dependency checks, as the aws_sdk_transcribe_streaming SDK already requires Python 3.12+.
• I have tested this implementation for 2 weeks, no error so far.