docs: update custom stream SOCKS5 example to modern API (fixes outdated callback usage)

[Priyank-Gaur]

This PR updates the “Connecting using custom stream” documentation.

The old example used a callback-style:
stream: function (cb) { ... }

This pattern no longer works in current versions of mysql2 and results in the runtime error:
TypeError: cb is not a function

What changed

Replaced the outdated callback-based SOCKS5 example.
Added a modern example where stream returns a Duplex stream or a Promise that resolves to one.
Added a short note explaining that the callback-style API is no longer supported.

Why this change

The existing docs mislead users into using a deprecated pattern that fails on current mysql2 versions.
The updated version shows the correct API and prevents confusion/errors for anyone connecting over SOCKS5 or other custom transport streams.

Related Issue
Fixes #3655

[wellwelwel]

Thanks, @Priyank-Gaur! ✨

[siddharthvp]

What versions of mysql2 and socks is this tested with?

With the latest ones (mysql2 3.15.3, socks 2.8.7):

Firstly, socks createConnection requires command to be passed. Without that it gives:

SocksClientError: An invalid SOCKS command was provided. Valid options are connect, bind, and associate.

If we set command to any of those 3 values, the code gives:

    this.stream.on('error', this._handleNetworkError.bind(this));
                ^
TypeError: this.stream.on is not a function

when getting a connection from the pool.

[Priyank-Gaur]

What versions of mysql2 and socks is this tested with?

With the latest ones (mysql2 3.15.3, socks 2.8.7):

Firstly, socks createConnection requires command to be passed. Without that it gives:

SocksClientError: An invalid SOCKS command was provided. Valid options are connect, bind, and associate.

If we set command to any of those 3 values, the code gives:

    this.stream.on('error', this._handleNetworkError.bind(this));
                ^
TypeError: this.stream.on is not a function

when getting a connection from the pool.

Thanks for checking this!

My PR focused on updating the old callback-style example to the modern stream-returning API since the previous example no longer worked with mysql2.

It looks like the SOCKS library (socks package) now requires an additional command field and may behave differently in the latest versions.

If needed, I can open a follow-up PR or If you want you can open a PR to update the example with a working configuration for the latest socks version.

[siddharthvp]

As I said in #3655, there is no working configuration I can come up with.