Tunnel interface overwrites existing files when creating Unix sockets #1140

Description

When a Host-to-Workshop tunnel is configured with a Unix socket endpoint, Workshop may overwrite an existing file at the specified path during launch.

This can cause unintended data loss, as the existing file is replaced with a Unix socket without any warning or error.

Steps to Reproduce

1. Configure the tunnel in .workshop/dev.yaml:

   name: dev
   base: [email protected]
   
   sdks:
     - name: system
       plugs:
         probe:
           interface: tunnel
           endpoint: $HOME/workshop-tunnel-test/target.sock
   
     - name: project-socket-probe

2. Define .workshop/socket-probe/sdk.yaml:

   name: socket-probe
   
   slots:
     probe:
       interface: tunnel
       endpoint: "@workshop-socket-probe"

3. Create an existing regular file and launch Workshop:

   echo "SENTINEL" > target.sock
   stat -c '%F %n' target.sock
   # regular file target.sock
   
   workshop launch --verbose
   
   stat -c '%F %n' target.sock
   # socket target.sock    <- overwrited

Expected Behavior

Workshop should reject tunnel creation if the Unix socket endpoint path already exists, regardless of the existing file type, rather than overwriting it.

Proposed Fix

Validate the endpoint path using os.Lstat() before configuring the LXD proxy device.

• Allow creation if the path does not exist.
• Reject creation if the path already exists, including regular files, directories, symbolic links, and Unix sockets.
• Return a clear error without modifying the existing file.

Verification

With the proposed fix, Workshop aborts launch with:

ERROR cannot create socket ".../target.sock": path already exists

The original file remains unchanged.

I have also verified that this fix does not affect the normal Workshop lifecycle. The start, stop, and refresh operations all continue to work as expected.