fix(aws): recover Nova Sonic system instability errors

[nightcityblade]

Summary

• treat Nova Sonic “System instability detected. Please retry your request.” ValidationException responses as recoverable stream failures
• route those errors through the existing realtime session restart path instead of emitting a fatal non-recoverable error
• add regression coverage for recoverable vs non-recoverable validation messages

Fixes #6010

Testing

• uv run --python 3.12 ruff check livekit-plugins/livekit-plugins-aws/livekit/plugins/aws/experimental/realtime/realtime_model.py livekit-plugins/livekit-plugins-aws/tests/test_realtime_validation_errors.py
• uv run --python 3.12 --package livekit-plugins-aws --extra realtime pytest livekit-plugins/livekit-plugins-aws/tests/test_realtime_validation_errors.py

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.

[tinalenguyen]

thanks for the pr!