#55115 added the dead-target registry so confirmed-dead delivery targets are
short-circuited. Its documented scope (gateway/dead_targets.py) is deliberately
narrow: only *whole-chat* deaths -- the `forbidden` and chat-level `not_found`
(`chat not found`) kinds -- should be recorded; "Thread/topic-level not_found is
NOT recorded here ... a deleted topic does not mean the parent chat is dead."
But the implementation doesn't honor that scope. classify_send_error collapses
chat-level "chat not found" AND thread/message-level not_found ("thread not
found", "topic_deleted", "message_id_invalid", "message to edit/reply not
found") into one "not_found" kind, _DEAD_ERROR_KINDS contains "not_found"
wholesale, and deliver()'s except marks the PARENT chat_id dead. So a single
deleted Telegram topic or edited-away message permanently marks the entire chat
(and every future scheduled / cron / agent delivery to it) dead -- silently. The
adapter self-heal the docstring relies on only covers the non-private-group
thread retry; named-DM-topic and message-level failures propagate to deliver()'s
except and wrongly kill the whole chat.
Add is_chat_level_not_found() (factoring the not_found substrings into chat-level
vs sub-chat-level constants) and gate the delivery dead-path: a "not_found" only
marks the target dead when it is chat-level. classify_send_error's public
contract is unchanged (still returns "not_found" for every shape); only the
mark_dead decision is refined, restoring the registry's documented scope.
Cross-platform: telegram/slack/discord delivery all flow through
classify_send_error -> mark_dead. Adds regression tests through the real
deliver() path plus helper/classifier units.
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