Regression introduced by #26609

Commit ede47a54b added _recover_telegram_topic_thread_id to pin inbound DM topic routing to the user's most-recent binding when the inbound thread_id is either lobby/General or unknown to existing bindings.

The "unknown → recover" branch is wrong: when a user creates a new Telegram DM topic, the inbound thread_id is real and non-lobby, but it has no binding yet because _record_telegram_topic_binding runs at line ~7876 — after recovery has already decided to redirect. The result: every message in the new topic is silently routed into the previously-active topic's session, and the new topic is never bound.

Live evidence from gateway logs:

telegram topic recovery: chat=1232992028 user=1232992028 '27453' -> 27374
telegram topic recovery: chat=1232992028 user=1232992028 '27453' -> 27374
telegram topic recovery: chat=1232992028 user=1232992028 '27453' -> 27374

27453 is the freshly-created topic id; 27374 is the previous session's topic. Every message keeps being hijacked because the binding for 27453 is never written.

The fix

The two cases that warrant recovery are:

1. thread_id is None / empty — _build_message_event stripped it ([Bug]: Telegram DM sends fail with 'Message thread not found' — spurious thread_id from reply chains #3206).
2. thread_id is a Telegram General/lobby magic id.

Both are lobby cases. A non-lobby thread_id always belongs to the topic the user intends — whether it is already bound or brand new. The fix collapses the recovery guard to is_lobby only and removes the known set lookup entirely.

Trade-off: a reply to a message in a deleted topic delivers a non-lobby but truly stale thread_id. Previously that would have been recovered; now it becomes an orphan message with no binding — consistent with what Telegram itself shows the user when the topic is gone.

Test changes

• test_recover_rewrites_unknown_thread_id_to_most_recent → renamed to test_recover_returns_none_for_unknown_thread_id and body updated to assert None (no hijack).
• Added test_recover_returns_none_for_brand_new_topic: bindings exist for topic 12345, inbound thread_id="99999" (non-lobby, unbound). Asserts None.
• All previously-passing tests (test_recover_returns_none_for_known_topic, test_recover_rewrites_lobby_thread_id_to_most_recent, test_recover_returns_none_when_topic_mode_disabled, test_recover_returns_none_when_no_bindings_yet) remain unchanged and pass.