What changed and why

Per the reporter's clarification on 2026-06-06, this restores the expected Telegram DM topic flow in two places.

• Make _recover_telegram_topic_thread_id() opt-in via gateway.platforms.telegram.extra.pin_root_dm_replies instead of always rewriting lobby-shaped root-DM replies to the most recent topic. This keeps fresh root-DM messages from being silently redirected into an older topic lane.
• Auto-enable Telegram topic mode when _schedule_telegram_topic_title_rename() sees a real DM topic thread for the first time, so the existing rename pipeline can run even if /topic was never explicitly invoked beforehand.
• Add regression coverage for both behaviors in tests/gateway/test_telegram_topic_mode.py.

Fixes #30411

Addressing maintainer feedback

This issue was previously tied to the #26609 / #28513 regression cluster, with #28605 called out as the canonical follow-up and #29287 / #29546 noted as duplicate fix PRs. The new 2026-06-06 followup added fresh, specific behavior that was still missing on main: opt-in root-DM pinning and lazy topic-mode seeding for auto-rename, so this PR implements that narrower follow-on instead of reopening any of those older branches.

How to test

• Activate the repo venv and run pytest tests/gateway/test_telegram_topic_mode.py -q.
• Verify test_recover_returns_none_without_opt_in_pinning keeps root-DM messages from being rebound when pin_root_dm_replies is unset.
• Verify test_recover_rewrites_lobby_thread_id_to_most_recent still passes when pin_root_dm_replies is explicitly enabled.
• Verify test_schedule_topic_rename_auto_enables_topic_mode_for_new_dm_topic proves the rename scheduler now seeds topic mode for a newly created DM topic lane.

What platforms tested on

• Local macOS worker environment (focused pytest coverage only)
• No live Telegram gateway manual run in this sandbox