Bug 507222

When using KDE Plasma 6, the freedesktop utility `xdg-open` does not correctly recognize or dispatch custom URL schemes registered via `x-scheme-handler` MIME types. Instead of opening the custom handler (e.g., a desktop file registered for scheme vivaldiopen://), `xdg-open` falls back to opening the default web browser (Firefox in my case).

This breaks expected usage of custom protocols and only affects `xdg-open`. Native KDE utilities such as `kioclient` and the `gio` command respect the custom handler and open the correct application.

Steps to reproduce:
1. Register a custom desktop file for a custom scheme, e.g. vivaldiopen://
2. Confirm KDE MIME apps list recognizes it (e.g., grep vivaldiopen ~/.config/mimeapps.list)
3. Run `kioclient exec "vivaldiopen://example.com"` → opens correct handler
4. Run `gio open "vivaldiopen://example.com"` → opens correct handler
5. Run `xdg-open "vivaldiopen://example.com"` → falls back to Firefox (default browser)

Workarounds:
- Using a wrapper script replacing xdg-open in ~/.local/bin/ that routes the custom scheme to kioclient works well.
- Aliasing xdg-open to gio open also works, but is less flexible.

Additional info:
-  xdg-utils version: 1.2.1-1
- Operating System: EndeavourOS (Arch Based)
- KDE Plasma Version: 6.4.3
- KDE Frameworks Version: 6.16.0
- Qt Version: 6.9.1
- Kernel Version: 6.15.6-zen1-1-zen (64-bit)
- Graphics Platform: X11

The issue appears to be caused by `xdg-open` not supporting KDE Plasma 6 properly. It does not handle KDE_SESSION_VERSION=6, and misclassifies custom URL schemes as normal HTTP/HTTPS URLs, causing fallback to the default browser.

Request:
Please update `xdg-open` or KDE’s integration to properly handle custom URL schemes in Plasma 6, respecting registered x-scheme-handler MIME types and dispatching accordingly.