fix(telegram): respect explicit private thread routing (#59453)

[teknium1]

Summary

Cron-resolved Telegram forum topics on positive chat IDs now route by message_thread_id instead of logging a false "requires a reply anchor" DM-topic failure.

Companion to #58165. Root cause: _is_private_dm_topic_send() OR'd a positive-chat-id + thread_id heuristic with the explicit telegram_dm_topic_reply_fallback flag, so cron forum-topic sends were misclassified as anchor-required DM sends.

Changes

• plugins/platforms/telegram/adapter.py: remove _looks_like_private_chat_id() and drop the positive-chat-id heuristic from _is_private_dm_topic_send(); anchor-required path is now gated solely on explicit telegram_dm_topic_reply_fallback metadata.

Genuine DM-topic sends are unaffected — gateway/platforms/base.py::_thread_metadata_for_source always sets telegram_dm_topic_reply_fallback=True for Telegram DM-chat threaded sends (gated on chat_type=="dm"), so the fail-loud contract holds without the chat-id-sign inference.

Validation

Case	Before	After
cron forum topic (positive chat + thread_id, no flag)	fail-loud (false failure)	routes via message_thread_id
DM-topic fallback (flag set, no anchor)	fail-loud	fail-loud (preserved)
explicit direct_messages_topic_id	routes direct, no anchor	routes direct, no anchor (preserved)

Tests: 118 passed (tests/gateway/test_telegram_thread_fallback.py +3 new, test_telegram_reply_mode.py, test_delivery.py). Standalone classification E2E confirmed all four cases.

Salvaged from #59453 by @herbalizer404 (authorship preserved).

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