#17968: fix(telegram): restore DM topic thread ids with send-path fallback

## Summary Restores DM topic reply targeting by including `message_thread_id` for `scope: "dm"` in `buildTelegramThreadParams()`. This keeps private-topic/thread UX working while preserving existing send-path fallback behavior (retry without thread id on Telegram rejection). ## Why `2026.2.15` omitted DM `message_thread_id` to avoid `400 Bad Request: message thread not found`. That fixed delivery failures, but can cause replies in Telegram private topics to appear in the main tab instead of the active topic. ## Changes - `src/telegram/bot/helpers.ts` - For `thread.scope === "dm"`, return `{ message_thread_id }` instead of omitting. - Keep General forum topic (`id=1`) omission behavior unchanged. - `src/telegram/bot/helpers.test.ts` - Update DM-thread expectations to include/normalize `message_thread_id`. ## Related - Follow-up regression report: #17960 - Prior fix: #10942 ## Notes I could not run the full test suite in this environment because project test deps/tooling are not fully installed in the sandbox PATH, but the updated unit tests reflect the new behavior. <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR re-enables `message_thread_id` for DM-scoped threads in `buildTelegramThreadParams`, restoring proper reply targeting in Telegram private topic/thread conversations. The prior fix (#10942) had omitted this field entirely to avoid `400 Bad Request: message thread not found` errors, but that caused replies to land in the main chat tab instead of the active topic. - `buildTelegramThreadParams` now returns `{ message_thread_id }` for `scope: "dm"` instead of `undefined` - Tests updated to assert thread ID inclusion and normalization for DM threads - The `send.ts` path has `withTelegramThreadFallback` to retry without the thread ID on rejection, but the `delivery.ts` path (bot reply delivery) does not have this fallback — this was already discussed in a prior review thread <h3>Confidence Score: 3/5</h3> - This PR is a targeted behavior change with a known gap in fallback coverage on one send path. - The code change itself is small and correct for its stated purpose. Tests are consistent with the implementation. However, the `delivery.ts` send path (used for bot reply delivery) lacks the `withTelegramThreadFallback` mechanism that `send.ts` has, meaning if a DM topic thread ID becomes stale, replies through `delivery.ts` will fail without retry. This was the exact scenario the prior fix was protecting against. The score of 3 reflects that the change works correctly for valid DM topics but reintroduces a partial regression risk on one delivery path. - `src/telegram/bot/delivery.ts` — uses `buildTelegramThreadParams` directly without `withTelegramThreadFallback`, meaning DM thread ID failures won't be retried on this path. _{Last reviewed commit: 17d256b} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->