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