fix: connect to backend immediately on channel active for server-first protocols(#794)

[Ronitsabhaya75]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

closes #794

TCP port forwarding currently fails for server-first protocols (SMTP, FTP, SSH, PostgreSQL) because the backend connection is only established when the client sends data.

Current behavior in ConnectHandler.swift:

func channelRead(context: ChannelHandlerContext, data: NIOAny) {
    if self.pendingBytes.isEmpty {
        self.connectToServer(context: context)  // Only connects on first data!
    }
}

This breaks any protocol where the server speaks first.

The fix:
Connect to the backend immediately when the channel becomes active, not when data arrives. This allows bidirectional communication from the start.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[liufeng-xiwo]

The problem also breaks connecting mysql service with port forwarding.

[Ronitsabhaya75]

The problem also breaks connecting mysql service with port forwarding.

can you go bit in detail please.

[jglogan]

@Ronitsabhaya75 Thanks for the simplification+fix! Let's get this into the release.

@liufeng-xiwo I don't know whether I've seen this issue with mysql. If you still have a problem there, feel free to create another issue and provide the steps that would allow us to reproduce the problem you see.