Add support for socks5h proxy scheme

[Copilot]

Description

HttpClient now accepts socks5h:// proxy URIs. Previously, attempts to use this scheme threw NotSupportedException with message "Only the 'http', 'https', 'socks4', 'socks4a' and 'socks5' schemes are allowed for proxies."

The socks5h scheme is a cURL convention indicating SOCKS5 with DNS resolution handled by the proxy. Our implementation treats it identically to socks5 since we already forward hostnames to the proxy by default.

Changes

• HttpUtilities.SocketsHttpHandler.cs: Added socks5h to IsSocksScheme()
• SocksHelper.cs: Added socks5h case to EstablishSocksTunnelAsync(), routing to SOCKS5 handler
• HttpEnvironmentProxy.cs: Added parsing for socks5h:// prefix in environment variables
• Strings.resx: Updated error message to include socks5h in supported schemes list
• Tests: Extended SocksProxyTest and HttpEnvironmentProxyTest to cover socks5h scheme

Example

using var handler = new SocketsHttpHandler();
handler.Proxy = new WebProxy("socks5h://username:password@proxy.example.com:1080");
using HttpClient client = new HttpClient(handler);
var response = await client.GetStringAsync("http://ifconfig.me/ip");

Type of change

• Bug fix (non-breaking change which fixes an issue)

Testing

Unit tests covering proxy scheme validation and environment variable parsing pass with new socks5h test cases included.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>HttpClient not support SOCKS5H proxy</issue_title>
<issue_description>### Description

   在 System.Net.Http.HttpConnectionPoolManager.SendAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
   在 System.Net.Http.HttpConnectionHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   在 System.Net.Http.HttpMessageHandlerStage.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   在 System.Net.Http.Metrics.MetricsHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   在 System.Net.Http.HttpMessageHandlerStage.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   在 System.Net.Http.DiagnosticsHandler.<SendAsyncCore>d__10.MoveNext()
   在 System.Runtime.CompilerServices.ConfiguredValueTaskAwaitable`1.ConfiguredValueTaskAwaiter.GetResult()
   在 System.Net.Http.RedirectHandler.<SendAsync>d__4.MoveNext()
   在 System.Net.Http.SocketsHttpHandler.<<SendAsync>g__CreateHandlerAndSendAsync|115_0>d.MoveNext()
   在 System.Net.Http.HttpClient.<GetStringAsyncCore>d__41.MoveNext()
   在 ConsoleApp1.Program.<Main>d__0.MoveNext() 在 C:\Users\ali\Desktop\ConsoleApp1\ConsoleApp1\Program.cs 中: 第 15 行


Only the 'http', 'https', 'socks4', 'socks4a' and 'socks5' schemes are allowed for proxies.

Reproduction Steps

using var handler = new SocketsHttpHandler();
handler.Proxy = new WebProxy("socks5h://username:password@ip:port");
using HttpClient client = new HttpClient(handler);
var response = await client.GetStringAsync("http://ifconfig.me/ip");

Expected behavior

normal request

Actual behavior

throw an exception

Only the 'http', 'https', 'socks4', 'socks4a' and 'socks5' schemes are allowed for proxies.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

@MihaZupan https://en.wikipedia.org/wiki/SOCKS > Since clients are allowed to use either resolved addresses or domain names, a convention from [cURL](https://en.wikipedia.org/wiki/CURL) exists to label the domain name variant of SOCKS5 "socks5h", and the other simply "socks5". A similar convention exists between SOCKS4a and SOCKS4.

TIL socks5h is a thing.
I think it'd be fine to add with the same behavior as our socks5 support, but I would not change our socks5 logic to resolve addresses locally. We can treat both the same and always forward the host to the proxy, the user always has the option to do the name resolution locally before making the request.</comment_new>

• Fixes HttpClient not support SOCKS5H proxy #123217

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[MihaZupan]

As long as tests are passing this LGTM

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Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.