chore: address review feedback

- Extract _safe_cast() helper for robust config parsing (TypeError + ValueError)
- Use async with for AsyncClient to prevent resource leaks
- Add os.makedirs for temp directory safety
- Add input validation for api_key, voice_id, and text length
- Normalize content-type header comparison
- Add tests for validation and invalid config fallbacks

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jaebong-human