Xubin Ren c88d97c652 fix: fall back to heuristic when bot open_id fetch fails
If _fetch_bot_open_id returns None the exact-match path would silently
disable all @mention detection. Restore the old heuristic as a fallback.
Add 6 unit tests for _is_bot_mentioned covering both paths.

Made-with: Cursor
2026-04-06 13:49:38 +08:00
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