feat(telegram): auto-discover fallback IPs via DoH when api.telegram.org is unreachable (#3376)

* feat(telegram): auto-discover fallback IPs via DoH when api.telegram.org is unreachable

On some networks (university, corporate), api.telegram.org resolves to a
valid Telegram IP that is unreachable due to routing/firewall rules. A
different IP in the same Telegram-owned 149.154.160.0/20 block works fine.

This adds automatic fallback IP discovery at connect time:
1. Query Google and Cloudflare DNS-over-HTTPS for api.telegram.org A records
2. Exclude the system-DNS IP (the unreachable one), use the rest as fallbacks
3. If DoH is also blocked, fall back to a seed list (149.154.167.220)
4. TelegramFallbackTransport tries primary first, sticks to whichever works

No configuration needed — works automatically. TELEGRAM_FALLBACK_IPS env var
still available as manual override. Zero impact on healthy networks (primary
path succeeds on first attempt, fallback never exercised).

No new dependencies (uses httpx already in deps + stdlib socket).

* fix: share transport instance and downgrade seed fallback log to info

- Use single TelegramFallbackTransport shared between request and
  get_updates_request so sticky IP is shared across polling and API calls
- Keep separate HTTPXRequest instances (different timeout settings)
- Downgrade "using seed fallback IPs" from warning to info to avoid
  noisy logs on healthy networks

* fix: add telegram.request mock and discovery fixture to remaining test files

The original PR missed test_dm_topics.py and
test_telegram_network_reconnect.py — both need the telegram.request
mock module. The reconnect test also needs _no_auto_discovery since
_handle_polling_network_error calls connect() which now invokes
discover_fallback_ips().

---------

Co-authored-by: Mohan Qiao <Gavin-Qiao@users.noreply.github.com>

tests/gateway/test_dm_topics.py

@@ -32,7 +32,7 @@ def _ensure_telegram_mock():
     telegram_mod.constants.ChatType.CHANNEL = "channel"
     telegram_mod.constants.ChatType.PRIVATE = "private"
 
-    for name in ("telegram", "telegram.ext", "telegram.constants"):
+    for name in ("telegram", "telegram.ext", "telegram.constants", "telegram.request"):
         sys.modules.setdefault(name, telegram_mod)