Runtime does not make KIO fuse paths accessible

Reproduce with Dolphin/KIO Fuse by creating a network mount, launching a game with Sniper, and checking the path from the game namespace with nsenter. Trace how the Sniper namespace handles /run/user/1000/kio-fuse-#####; done means the mounted network path is accessible to the game and the file picker can load a file from it.

Your system information

• Steam Runtime Version: 0.20250718.0+srt1
• Distribution (e.g. Ubuntu 18.04): EndeavourOS Linux 6.16.7-1-cachyos
• Link to your full system information (Help -> Steam Runtime Diagnostics) in a Gist: https://gist.github.com/VasilisThePikachu/edee9aa88ece462bd6ffa2750b2f973f
• Have you checked for system updates?: YES
• What compatibility tool are you using?: Sniper
• What versions are listed in steamapps/common/SteamLinuxRuntime/VERSIONS.txt?

#Name   Version         Runtime Runtime_Version Comment
depot   0.20240806.0                    # Overall version number
LD_LIBRARY_PATH -       scout   -       # see ~/.steam/root/ubuntu12_32/steam-runtime/version.txt
scripts 0.20240806.0                    # from steam-runtime-tools

• What versions are listed in steamapps/common/SteamLinuxRuntime_soldier/VERSIONS.txt?

#Name   Version         Runtime Runtime_Version Comment
depot   2.0.20250826.159137                     # Overall version number
pressure-vessel 0.20250820.0    scout           # pressure-vessel-bin.tar.gz
scripts 0.20250820.0                    # from steam-runtime-tools
soldier 2.0.20250826.159137     soldier 2.0.20250826.159137     # soldier_platform_2.0.20250826.159137/

• What versions are listed in steamapps/common/SteamLinuxRuntime_sniper/VERSIONS.txt?

#Name   Version         Runtime Runtime_Version Comment
depot   3.0.20250826.159138                     # Overall version number
pressure-vessel 0.20250820.0    scout           # pressure-vessel-bin.tar.gz
scripts 0.20250820.0                    # from steam-runtime-tools
sniper  3.0.20250826.159138     sniper  3.0.20250826.159138     # sniper_platform_3.0.20250826.159138/

Please describe your issue in as much detail as possible:

Mounts made with KIO fuse (Such as network mounts using KDE's Dolphin) are unavailable inside Sniper, while the file picker will be able to see them, games will complain about inaccessible files when trying to pick and load them.

Using nsenter to go inside the sniper namespace of the running game shows that /run/user/1000/kio-fuse-##### is not mounted within, which explains the error.

In this case, the game I was testing was Space Station 14 and trying to play midi files from a network share. But while it is visible within the dolphin file picker screen, the game is unable to read the file as the path is not actually available.

Steps for reproducing this issue:

1. Install dolphin (KDE's default file manager)
2. Create a network mount via the network tab (type does not matter)
3. Run a game with sniper (The game needs to have file picker support.)
4. Within the game try to pick a file within the network share
5. The game should complain about being unable to find the file/part of the path