fix: Proxy-Authorization header silently stripped by axios

[DennisGaida]

Summary

In this pull request, the following changes are made:

• axios's Node HTTP adapter unconditionally strips any header named Proxy-Authorization unless a real upstream proxy is configured (removeProxyAuthorization in lib/adapters/http.js), even when the user explicitly set it via the monitor's Headers field. This breaks the common Traefik forward-auth + Authelia pattern of using Proxy-Authorization for non-interactive service-account bypass (distinct from Authorization, which is reserved for the backend app).
  • This is actually a regression, but I didn't dig into when this change happened - I have been using Proxy-Authorization via the Header field in uptime-kuma for a while, but at some point this stopped working and I just got around to investigating why this was.
• Reinjects the header directly on the outgoing request via a wrapped agent.addRequest, right before dispatch and after axios has already sanitized its own options.headers — so it reaches the wire regardless of axios's internal handling.
• Adds HTTP Basic Auth (Proxy-Authorization) (id: proxy-basic) as a new Authentication method in the dropdown, reusing the existing basic_auth_user/basic_auth_pass fields (no schema change) so this is discoverable instead of requiring a raw custom-header workaround. I am completely open to just naming this "Proxy-Authorization" instead of HTTP Basic Auth (Proxy-Authorization)", wasn't sure what is better.

Fixes #7772

Please follow this checklist to avoid unnecessary back and forth (click to expand)

• ⚠️ If there are Breaking change (a fix or feature that alters existing functionality in a way that could cause issues) I have called them out
• 🧠 I have disclosed any use of LLMs/AI in this contribution and reviewed all generated content.
  I understand that I am responsible for and able to explain every line of code I submit.
• 🔍 Any UI changes adhere to visual style of this project.
• 🛠️ I have self-reviewed and self-tested my code to ensure it works as expected.
• 📝 I have commented my code, especially in hard-to-understand areas (e.g., using JSDoc for methods).
• 🤖 I added or updated automated tests where appropriate.
• 📄 Documentation updates are included (if applicable).
• 🧰 Dependency updates are listed and explained.
• ⚠️ CI passes and is green.

Status / what's still open

• Backend fix (addRequest reinjection + new proxy-basic auth method) verified against a real production Authelia/Traefik forward-auth deployment: header now reaches the destination, monitor reports UP again.
• Have tested end-to-end: Using {"Proxy-Authorization": "Basic xxx"} does not produce the header (eaten by axios), using the new "HTTP Basic Auth (Proxy-Authorization)" choice for authentication, produces the header and I am authenticated.
• No automated test added yet for the addRequest reinjection specifically — open to suggestions on the best way to cover this given it touches raw agent/socket behavior rather than something easily mockable at the axios-config level - and also I am not sure whether other libraries than axios would exhibit the same behavior.

Screenshots for Visual Changes

I'm not sure whether screenshots of a new dropdown value are necessary, because the juice is in the backend (axios), but anyways - here is the new option:

and here is the option including the existing (basic auth) fields:

[github-actions]

Thanks for the PR! If anyone would like to help with testing, run: npx kuma-pr DennisGaida:fix/proxy-authorization-header-stripped (requires Node.js and Docker)