feat: add ConnectionCheck utility for connection diagnostics

[kNoAPP]

Summary

This PR ports the ConnectionCheck connection-diagnostics utility from client-sdk-js to Dart, the same checks that power livekit.io/connection-test. Flutter apps can now run structured preflight diagnostics proactively (at app start) or reactively (when room.connect() fails) to determine why a connection cannot be established (firewall, VPN, blocked TURN, etc.), instead of asking users to reproduce on web.

Ref: Diagnosing Connection Errors with Connection Test Utility

Demo

API

final connectionCheck = ConnectionCheck(url, token);
final listener = connectionCheck.createListener();
listener.on<ConnectionCheckUpdateEvent>((event) {
  print('${event.info.name}: ${event.info.status.name}');
});

// recommended minimum set
await connectionCheck.checkWebsocket();
await connectionCheck.checkWebRTC();
await connectionCheck.checkTURN();

// additional checks
await connectionCheck.checkReconnect();
await connectionCheck.checkPublishAudio();
await connectionCheck.checkPublishVideo();
await connectionCheck.checkConnectionProtocol();
await connectionCheck.checkCloudRegion();

print('all checks passed: ${connectionCheck.isSuccess}');
await listener.dispose();
await connectionCheck.dispose();

Testing

• flutter test: 364 passing (20 new tests covering the Checker base lifecycle, ConnectionCheck orchestration/eventing, ICE candidate parsing, and WebSocketCheck against the mock WebSocket connector).
• flutter analyze, dart format, import_sorter, check_version passing
• Manually tested web, iOS, Android

Example

example/lib/pages/connection_check.dart adds a "Connection Check" page (launched from the connect screen) that runs all checks and displays per-check status + logs.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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