Add WPAD protocol explanation to HTTP proxy auto-detection docs

[Copilot]

The automatic-proxy-detection.md article was removed and now redirects to httpclient.md#configure-an-http-proxy, but the target section gave no practical explanation of how Windows automatic proxy detection actually works.

Changes

• docs/fundamentals/networking/http/httpclient.md — Added a concise explanation of the WPAD (Web Proxy Auto-Discovery) protocol under the "Use a global default proxy" section, clarifying that when Automatically detect settings is enabled on Windows, the system queries DNS for a host named wpad and fetches a PAC file from http://wpad/wpad.dat to determine the correct proxy per URL.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>http://wpad/wpad.dat</issue_title>
<issue_description>Step 4 could mention try visit http://wpad/wpad.dat to load the configuration file, so readers experience Eureka how auto-proxys work. ”DNS is queried for a host with WPAD as its name or alias” is functionally correct but summary the full WDAP protocol in one single sentence,which makes it hard to understand.

This legacy MSDN article is very useful:
https://docs.microsoft.com/en-us/archive/msdn-magazine/2005/august/take-the-burden-off-users-with-automatic-configuration-in-net

---

Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: a66edea4-6660-f670-fe9d-94302ac98cb3
• Version Independent ID: 24143fa6-d272-f5d6-eef4-c05f6497d99e
• Content: Automatic Proxy Detection
• Content Source: docs/framework/network-programming/automatic-proxy-detection.md
• Product: dotnet-framework
• Technology: dotnet-networking
• GitHub Login: @karelz
• Microsoft Alias: ncldev</issue_description>

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

@karelz @perrog do you mean adding something like "(looking for file http://wpad/wpad.dat)" in step [3]? (not 4 as you mentioned) I am not familiar with WDAP protocol, but if there is confirmation somewhere, this is the URL that is always searched for (and there are no alternatives/configurations), I wouldn't mind mentioning it here. Going into depth of WDAP protocol is IMO beyond the scope of this article.

cc @davidsh</comment_new>
<comment_new>@karelz
Is it something you would recommend to link from the article? Or maybe apply some of the suggestions above @davidsh?</comment_new>

• Fixes http://wpad/wpad.dat #18231

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

---

Internal previews

📄 File	🔗 Preview link
docs/fundamentals/networking/http/httpclient.md	Make HTTP requests with the HttpClient class

[BillWagner]

This is ready for final review.

[Copilot]

Pull request overview

This PR updates the HTTP proxy auto-detection guidance in httpclient.md by adding a short explanation of how Windows automatic proxy detection works (WPAD/PAC), since the previous standalone article now redirects into this page.

Changes:

• Add a WPAD/PAC explanation under Use a global default proxy to describe how Windows auto-detects proxy settings.
• Update ms.date and add ai-usage: ai-assisted frontmatter metadata.