Fixes #18974.

What changed:

• Removed blanket suppression of message_thread_id for private chats in Telegram send paths.
• Preserved message_thread_id for DM topics.
• Kept existing fallback: on message thread not found, retry once without message_thread_id.
• Added guard comment to prevent reintroducing this regression.

Tests:

• Updated src/telegram/send.test.ts to assert DM topic sends keep message_thread_id.
• Added regression test to ensure chat not found does not trigger thread fallback retry.
• pnpm vitest run src/telegram/send.test.ts passes.

Greptile Summary

This PR fixes Telegram DM topic routing by removing the blanket suppression of message_thread_id for private chats (positive chat IDs). Previously, isTelegramPrivateChat stripped thread IDs for all positive chat IDs, which broke DM topics since Telegram Bot API does support topics in private chats. The fix replaces sign-based filtering with scope-based routing: parseTelegramTarget provides chatType, which buildTelegramThreadReplyParams uses to set the correct thread scope ("dm" vs "forum"), delegating to the existing buildTelegramThreadParams helper that already handled DM scope correctly.

• Removed isTelegramPrivateChat helper and its blanket thread ID suppression for positive chat IDs
• Added chatType parameter to buildTelegramThreadReplyParams to route thread scope by parsed chat type rather than chat ID sign
• Preserved the existing withTelegramThreadFallback retry for "message thread not found" errors, which safely handles plain DMs without topic support
• Added explicit guard (AIDEV-NOTE) against widening the fallback to cover "chat not found" errors
• Comprehensive test coverage: DM topic send, DM thread-not-found retry, and chat-not-found non-retry regression tests

Confidence Score: 5/5

• This PR is safe to merge — it fixes a clear routing regression with well-scoped changes and comprehensive test coverage.
• The change is minimal and focused: it removes an incorrect blanket suppression, replaces it with scope-based routing that leverages existing infrastructure (buildTelegramThreadParams already handled "dm" scope), and the thread-not-found retry fallback provides a safety net for edge cases. Three new regression tests cover DM topic sends, DM thread-not-found retry, and chat-not-found non-retry. No new dependencies, no security surface changes, and backward compatible (only changes behavior for DM targets with thread IDs, which were previously broken).
• No files require special attention.

_{Last reviewed commit: 3537aa9}