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>

gateway/config.py

@@ -601,6 +601,14 @@ def _apply_env_overrides(config: GatewayConfig) -> None:
             config.platforms[Platform.TELEGRAM] = PlatformConfig()
         config.platforms[Platform.TELEGRAM].reply_to_mode = telegram_reply_mode
     
+    telegram_fallback_ips = os.getenv("TELEGRAM_FALLBACK_IPS", "")
+    if telegram_fallback_ips:
+        if Platform.TELEGRAM not in config.platforms:
+            config.platforms[Platform.TELEGRAM] = PlatformConfig()
+        config.platforms[Platform.TELEGRAM].extra["fallback_ips"] = [
+            ip.strip() for ip in telegram_fallback_ips.split(",") if ip.strip()
+        ]
+
     telegram_home = os.getenv("TELEGRAM_HOME_CHANNEL")
     if telegram_home and Platform.TELEGRAM in config.platforms:
         config.platforms[Platform.TELEGRAM].home_channel = HomeChannel(