TF-3976 Support DNS SRV resolvers without Cloudflare/Google dependency

[dab246]

Issue

#3976

Description

This PR introduces improvements to the DNS SRV resolution flow by removing the hard dependency on Cloudflare and Google resolvers.
It enhances DnsLookupManager to use a multi-priority fallback mechanism including system DNS, public open resolvers (UDP/TCP), and DNS-over-HTTPS (DoH) providers such as Quad9, AdGuard, OpenDNS, and Yandex.

The goal is to increase privacy, resilience, and independence from GAFAM-controlled infrastructure, especially in restrictive or public network environments.

Changes

• Refactored DnsLookupManager to support prioritized resolver strategy:

1. System DNS (default)
2. Public Open DNS (UDP/TCP)
3. Public DoH (Quad9, AdGuard, OpenDNS, Yandex)
4. Cloud DNS (Google/Cloudflare) — now last fallback only

• Added timeout (3s) per resolver to prevent blocking calls
• Improved logging and error handling for each DNS layer
• Added full unit test coverage with mocked resolvers
  • Success path
  • Fallback logic
  • Timeout handling
  • Invalid email validation

Resolved

• Demo:

Screen_recording_20251028_112719.webm

• Logs

I/flutter (19422): [SystemUdpSrvClient] ⚠️ No system DNS found — let higher-level resolver fallback to public DNS.
I/flutter (19422): [SystemUdpSrvClient] Using 0 system DNS servers: 
I/flutter (19422): [BaseSrvResolver] ⚠️ No DNS servers configured.
I/flutter (19422): [TwakeMail] DnsLookupManager::lookupJmapUrl → ⚠️ No records via System Default, continuing...
I/flutter (19422): [TwakeMail] DnsLookupManager::lookupJmapUrl → 🔍 Trying Public DNS (UDP/TCP) (timeout: 3s)...
I/flutter (19422): [BaseSrvResolver] 🌐 Querying _jmap._tcp.linagora.com via 8 servers...
I/ImeTracker(19422): system_server:15b4d912: onCancelled at PHASE_CLIENT_ON_CONTROLS_CHANGED
I/flutter (19422): [BaseSrvResolver] ✅ 149.112.112.112 responded via UDP
I/flutter (19422): [TwakeMail] DnsLookupManager::lookupJmapUrl → ✅ Success via Public DNS (UDP/TCP): jmap.linagora.com

I/flutter (19422): [SystemUdpSrvClient] ⚠️ No system DNS found — let higher-level resolver fallback to public DNS.
I/flutter (19422): [SystemUdpSrvClient] Using 0 system DNS servers: 
I/flutter (19422): [BaseSrvResolver] ⚠️ No DNS servers configured.
I/flutter (19422): [TwakeMail] DnsLookupManager::lookupJmapUrl → ⚠️ No records via System Default, continuing...
I/flutter (19422): [TwakeMail] DnsLookupManager::lookupJmapUrl → 🔍 Trying Public DNS (UDP/TCP) (timeout: 3s)...
I/flutter (19422): [BaseSrvResolver] 🌐 Querying _jmap._tcp.stg.lin-saas.com via 8 servers...
I/flutter (19422): [BaseSrvResolver] ✅ 94.140.15.15 responded via UDP
I/flutter (19422): [TwakeMail] DnsLookupManager::lookupJmapUrl → ✅ Success via Public DNS (UDP/TCP): jmap.stg.lin-saas.com

[github-actions]

This PR has been deployed to https://linagora.github.io/tmail-flutter/4129.