Proxy blindly prepends base path to HTTP redirects, causing double-path 404s

[StickyDoughnut]

Bug description

In version 4.5.0, jupyter-server-proxy aggressively rewrites HTTP 302 Location headers returned by a proxied application, even if the application already included the proxy path in its absolute redirect URL.

This results in a duplicated path and a 404 error. This breaks applications like Label Studio, which require their external base URL to be explicitly defined via environment variables so that static assets load correctly.

This is a regression. In version 4.4.0, this exact setup works perfectly without duplicating the redirect path.

How to reproduce

1. Start a JupyterLab environment (e.g., inside Kubeflow).
2. Install jupyter-server-proxy==4.5.0 and label-studio in a virtual environment.
3. Export the application's required host URL using the proxy's absolute path:
   export LABEL_STUDIO_HOST="https://<jupyter-domain>/notebook/<namespace>/<notebook-name>/proxy/8080/"
4. Start the application in the terminal:
   label-studio start --port 8080
5. Open a browser and navigate to the base URL:
   https://<jupyter-domain>/notebook/<namespace>/<notebook-name>/proxy/8080/
6. The application receives the unauthenticated request and generates an HTTP 302 redirect to its login page:
   Location: https://<jupyter-domain>/notebook/<namespace>/<notebook-name>/proxy/8080/user/login/

Expected behaviour

The proxy should recognize that the Location header is already an absolute URL pointing to the correct proxy path (or correctly handle the app's X-Forwarded-Host headers). The user should land cleanly on:
https://<jupyter-domain>/notebook/<namespace>/<notebook-name>/proxy/8080/user/login/

Actual behaviour

The proxy intercepts the redirect and blindly prepends its routing prefix to the URL again, sending the user to a duplicated 404 path:
https://<jupyter-domain>/notebook/<namespace>/<notebook-name>/proxy/8080/notebook/<namespace>/<notebook-name>/proxy/8080/user/login/

[manics]

#544
I think the old behaviour was a bug, if your application knows the full prefix you should run it in absolute mode (set absolute_url=True)