Follow-up to #31501. When the send-fallback prune removes a chat's
final telegram_dm_topic_bindings row, also flip
telegram_dm_topic_mode.enabled to 0 in the same transaction.
Without this, a user who turns topics off in the Telegram client
(rather than via /topic off) leaves enabled=1 with zero lanes:
_recover_telegram_topic_thread_id keeps treating the chat as
topic-enabled and lobby messages keep hunting for bindings that no
longer exist. Clearing the flag makes recovery fully stand down once
the dead topics are gone.
Adds 3 regression tests covering the last-binding clear, the
multi-binding no-op, and the unmatched-prune no-op.
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
Teknium
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