Report a transport connection's failure the way a stream connection's is reported

[MDA2AV]

Context: integrating the experimental transport seam on ioxide (io_uring runtime) at https://github.com/MDA2AV/ioxide/tree/main/src/clients/ioxide.redis.stackexchange

Issue: when not using the stock transport, there is no notification when a connection dies, ConnectionFailed is not raised and commands awaiting a reply are cancelled insterad of failing with RedisConnectionException.

How this PR fixes it: RecordConnectionFailed only reports failure when _ioStream is set, adding _transportAdopted for when another transport is used instead.

Some more context:

IoxideTunnel.ConnectTransportAsync returns IoxideTransport so StackExchange.Redis takes the transport path and not the stream path, _ioStream stay null for the connection's life.
When the connection dies the check sees that _ioStream is null so the reporting is skipped.

[mgravell]

LGTM; I'm interested how ioxide has found this - I'm assuming this is broadly similar to how SocketSet spikes this, but if ioxide does it better: great!

[mgravell]

@MDA2AV also: to emphasize - this API is deliberately [Experimental] precisely because I'm very interested in any "this doesn't fit" etc feedback; I used SocketSet to do a random test, but if you have feedback: throw it at me! Also, if you're looking for things to test against: GarnetServer also has a fairly extensible network API - see SocketSet.Garnet - it might be of interest to you.