Improvements from our earlier #8269 salvage work applied to #7616:
- Platform token lock: acquire_scoped_lock/release_scoped_lock prevents
two profiles from double-connecting the same QQ bot simultaneously
- Send retry with exponential backoff (3 attempts, 1s/2s/4s) with
permanent vs transient error classification (matches Telegram pattern)
- Proper long-message splitting via truncate_message() instead of
hard-truncating at MAX_MESSAGE_LENGTH (preserves code blocks, adds 1/N)
- REST-based one-shot send in send_message_tool — uses QQ Bot REST API
directly with httpx instead of creating a full WebSocket adapter per
message (fixes the connect→send race condition)
- Use shared strip_markdown() from helpers.py instead of 15 lines of
inline regex with import-inside-method (DRY, same as BlueBubbles/SMS)
- format_message() now wired into send() pipeline
8d545da3ff