[Bug]: Tailscale spawn ENOTDIR breaks advertised desktop endpoints #5111

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/desktop

Steps to reproduce

1. On macOS, add a file rather than a directory to PATH. For example:

   export PATH="$PATH:/opt/homebrew/bin/docker-credential-ecr-login"

2. Start T3 Code Desktop with that environment.

3. If Network access is not already enabled from an earlier session, enable it in Settings → Connections.

4. Open or return to Settings → Connections.

The page loads advertised endpoints automatically and displays the error.

Expected behavior

The Connections panel should continue to show the loopback and LAN endpoints.

If Tailscale cannot start, T3 Code should treat Tailscale endpoints as unavailable rather than reject the whole advertised-endpoints request.

Actual behavior

The Connections panel shows:

Failed to load advertised desktop endpoints.

The desktop main-process log contains:

Error occurred in handler for 'desktop:get-advertised-endpoints': Error: spawn ENOTDIR
  errno: -20
  code: 'ENOTDIR'
  syscall: 'spawn'

Node resolves tailscale by walking PATH. When it reaches an entry that is a file rather than a directory, child_process.spawn can throw ENOTDIR.

The Effect child-process wrapper captures this synchronous throw as a defect. The Tailscale command code maps typed spawn failures, but it does not convert this defect into the existing TailscaleCommandSpawnError. The desktop fallback also handles typed failures only, so the defect escapes the IPC handler.

This affects both Tailscale status reads and Tailscale Serve commands, which use separate spawn paths.

Impact

Minor bug or occasional failure

The Connections endpoint list becomes unavailable for the affected environment, even though the loopback and LAN endpoints can still be calculated without Tailscale.

Version or commit

Observed in an installed macOS desktop build. The failure path was also validated against main at 61514c1294ff79e2a9924551b2042fcc0d1c3a34.

Environment

macOS, T3 Code Desktop, with a non-directory entry in PATH and Network access restored from saved settings.

Logs or stack traces

Error occurred in handler for 'desktop:get-advertised-endpoints': Error: spawn ENOTDIR
  errno: -20
  code: 'ENOTDIR'
  syscall: 'spawn'

Screenshots, recordings, or supporting files

None.

Workaround

Remove file entries from PATH, or ensure the directory containing the Tailscale executable appears before the invalid entry.

Disabling Network access also avoids the Tailscale status lookup, but removes the network-accessible endpoint.