tool_communication.py: Add error handling for non-existing parent

[urfeex]

When passing a path as a local device name where the parent directory doesn't exist, socat complains about the link target not existing. From that it isn't necessary clear that this means that the parent directory doesn't exist. This commit adds a check for that case explicitly with a clear error message.

Without this PR

Without the changes from this PR passing a non-existing parent folder will look like this:

$ scripts/tool_communication.py --device-name /tmp/path/doesnt/exist/ttyUR 192.168.178.103
[INFO] Robot IP: 192.168.178.103
[INFO] TCP Port: 54321
[INFO] Configuring PTY symlink at '/tmp/path/doesnt/exist/ttyUR'
[INFO] Starting socat with following command:
socat pty,link=/tmp/path/doesnt/exist/ttyUR,raw,ignoreeof,waitslave tcp:192.168.178.103:54321
2026/03/30 11:19:16 socat[59904] E symlink("/dev/pts/11", "/tmp/path/doesnt/exist/ttyUR"): No such file or directory
[INFO] Socat terminated

With this PR

With the changes from this PR passing a non-existing parent folder will look like this:

$ scripts/tool_communication.py --device-name /tmp/path/doesnt/exist/ttyUR 192.168.178.103
[INFO] Robot IP: 192.168.178.103
[INFO] TCP Port: 54321
[ERROR] Parent directory '/tmp/path/doesnt/exist' does not exist.
Socat needs an existing directory to create the PTY symlink.
Fix:
  - Create the parent directory, e.g. 'mkdir -p /tmp/path/doesnt/exist'.
  - Use a different device name with an existing parent directory.
[INFO] Exiting tool communication script.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.90%. Comparing base (4740f63) to head (e801e30).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
scripts/tool_communication.py	86.66%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #477      +/-   ##
==========================================
- Coverage   77.04%   76.90%   -0.14%     
==========================================
  Files         110      111       +1     
  Lines        5824     5902      +78     
  Branches     2577     2576       -1     
==========================================
+ Hits         4487     4539      +52     
- Misses        997     1023      +26     
  Partials      340      340              

Flag	Coverage Δ
python_scripts	75.90% <86.66%> (?)
start_ursim	83.08% <ø> (-1.48%)	⬇️
ur20-latest	72.29% <ø> (-0.01%)	⬇️
ur5-3.14.3	72.11% <ø> (-0.16%)	⬇️
ur5e-10.11.0	66.16% <ø> (+0.20%)	⬆️
ur5e-10.12.0	66.99% <ø> (-0.32%)	⬇️
ur5e-10.7.0	65.13% <ø> (-0.34%)	⬇️
ur5e-5.9.4	72.57% <ø> (-0.28%)	⬇️

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[srvald]

Looks good to me.

Thank you for improving the error handling by switching to exceptions and for adding tests.
The additional check ensuring that the parent directory exists makes sense and is something I hadn't taken into account, so thanks for adding that error case.