Private chats (positive numeric chat IDs) never support forum topics.
Sending message_thread_id to a private chat causes Telegram to reject
the request with '400: Bad Request: message thread not found', silently
dropping the message.
Guard all three send functions (sendMessageTelegram, sendStickerTelegram,
sendPollTelegram) to omit thread-related parameters when the target is a
private chat.
Root cause: the auto-reply pipeline can set messageThreadId from a
previous forum-group context, then reuse it when sending a DM.
Tests: add private-chat suppression assertions; update existing thread-
retry tests to use group chat IDs so the retry path is still exercised.
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