On a dual-stack network Plezy pinned API traffic and the notification
websocket to IPv4 while the native Plex apps and the media stream used
IPv6. dart:io looks up A before AAAA and connects in arrival order, so
the AAAA address was never tried when the IPv4 path answered within
250 ms.
Install an HttpClient.connectionFactory that does one lookup (keeping
the resolver's RFC 6724 order), interleaves families per RFC 8305 and
races candidates 250 ms apart. The task is returned before the lookup
starts so connectionTimeout and cancel cover resolution. TLS is done in
the factory since the SDK skips it once one is installed; proxied
requests stay plain. The library-event and companion-remote websockets
get the same client because WebSocket.connect otherwise builds its own.
Supersedes #2233.
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