Cherry-picked from PR #6894 by SHL0MS with fixes:
- Only batch TEXT messages; commands/media dispatch immediately
- Use build_session_key() for proper session-scoped batch keys
- Consistent naming (_text_batch_delay_seconds)
- Proper Dict[str, MessageEvent] typing
Discord splits at 2000 chars (lowest of all platforms). Adaptive delay
waits 2.0s when a chunk is near the limit, 0.6s otherwise.