Add send_message(text, parse_mode) method that can be used for both
notifications and command responses. Refactor _send_notification to
use the new method.
Changes:
- Add send_message() method with return value for success/failure
- Refactor _send_notification() to call send_message()
- Add comprehensive tests for send_message()
- Coverage: 93% for telegram_client.py
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>