SocketsHttpHandler only checks for 200 OK instead of 2XX for CONNECT responses

[MihaZupan]

https://www.rfc-editor.org/rfc/rfc9110.html#section-9.3.6-7

Any 2xx (Successful) response indicates that the sender (and all inbound proxies) will switch to tunnel mode immediately after the response header section

https://www.rfc-editor.org/rfc/rfc9112.html#section-6.3-2.2

Any 2xx (Successful) response to a CONNECT request implies that the connection will become a tunnel immediately after the empty line that concludes the header fields. A client MUST ignore any Content-Length or Transfer-Encoding header fields received in such a message.

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnection.cs

Line 787 in 39a31f0

else if (request.Method.IsConnect && response.StatusCode == HttpStatusCode.OK)

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.cs

Line 779 in 39a31f0

if (tunnelResponse.StatusCode != HttpStatusCode.OK)

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Stream.cs

Line 657 in 39a31f0

if (statusCode == 200 && _response.RequestMessage!.IsExtendedConnectRequest)

Just from reading the RFC, not sure we've seen a proxy that uses a different 2XX response.

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