Thirteen tests across four layers:
* ``SessionDB.delete_telegram_topic_binding`` — pin the new
helper's contract: removes only the (chat_id, thread_id) row
it was asked about, leaves siblings alone, returns 0 silently
when the row never existed, and is a no-op on a pristine
database whose topic-mode tables haven't been migrated yet.
* ``TelegramAdapter._prune_stale_dm_topic_binding`` — the glue
must drop the binding when ``self._session_store._db``
exposes the helper, swallow exceptions so a failed cleanup
never breaks the user-facing send, and refuse to issue a
DELETE for ``chat_id=None`` / ``thread_id=None`` so a
bookkeeping miss can't accidentally null-match every row.
* Source-level guards on ``TelegramAdapter.send`` and
``_send_message_with_thread_fallback`` — the prune call must
sit beside the two existing "Thread X not found, retrying
without message_thread_id" warnings, before the retry runs,
so a future refactor can't silently drop the cleanup wire.
* End-to-end semantic — once a topic is pruned, the
``GatewayRunner._recover_telegram_topic_thread_id`` walk
steers future inbound messages to the surviving binding
instead of the dead one. This is the exact behaviour change
the bug report's reproduction asks for: no more landings in
the wrong topic until the operator hand-edits ``state.db``.
Refs #31501
xxxigm
11246dbe21