fix(telegram): request guest_message in allowed_updates for guest mode

[elphamale]

Summary

• Guest mode (Bot API 10.0 `guest_message` @mentions from chats the bot isn't a member of) wasn't working: `allowed_updates=Update.ALL_TYPES` at both `start_polling()`/`start_webhook()` call sites tells Telegram to send every update type PTB has a typed field for — but PTB doesn't have one for `guest_message` yet, so it's excluded from that hardcoded list and Telegram drops it server-side before it ever reaches the process.
• No exception, no log line, nothing — the `_handle_guest_message_update` TypeHandler registers fine and just never fires, from any chat, indistinguishable from a backend outage.
• Confirmed live via `getWebhookInfo`: `allowed_updates` was missing `guest_message` even though `getMe` reports `supports_guest_queries: true`.
• Note on history: `git log -S` shows this exact `allowed_updates=Update.ALL_TYPES` line has been present unchanged since the 2026-06-20 platform-adapter migration (`5600105`), with `python-telegram-bot` pinned at `22.6` throughout — so this isn't a new regression from a dependency bump. It's unclear exactly how long guest mode was actually broken for in practice; treat this as "found broken and fixed today," not a claim about when it started.
• Fix: explicitly append `"guest_message"` to the allowed_updates list at both call sites, via a shared `_ALLOWED_UPDATES_WITH_GUEST` constant (getattr-guarded against `Update.ALL_TYPES` for test doubles that stub `Update` minimally).
• No test coverage existed for `allowed_updates` at all before this, which is how it shipped unnoticed. Added a regression test pinning the constant's content and that no call site bypasses it.

Test plan

• New regression test: `tests/gateway/test_telegram_guest_message_allowed_updates.py` (2 tests)
• Full `tests/gateway/` sweep via `run_tests_parallel.py`: clean except one pre-existing, unrelated Honcho mtime-cache test that also fails on pristine `origin/main`
• Verified live post-deploy: `getWebhookInfo` on the running bot now shows `guest_message` in `allowed_updates`

[elphamale]

Folded into #56476 (the PR whose own commit introduced the gap this fixes — see that PR's own commit history for the fixup) rather than shipping the feature broken and following up separately. #56477 rebased on top to keep the stack intact. Closing this one.