Fix serial port listing with unknown serial subsystems (Linux)

[puddly]

Caught (again) in https://github.com/home-assistant/core/actions/runs/24216216900/job/70698025832?pr=167615.

                    product=None,
                    bcd_device=None,
                    interface_description=None,
                    interface_num=None,
                )
            else:
                LOGGER.warning(
                    "Unknown serial device subsystem %r for device %r",
                    subsystem,
                    device,
                )
    
>           results.append(info)
                           ^^^^
E           UnboundLocalError: cannot access local variable 'info' where it is not associated with a value

A follow-up PR will enable as much of mypy as we can to catch things like this in the future.

---

The underlying issue is that Path.resolve() on a nonexistent path will return the same path unless called with strict=True. The GitHub Actions runner VM seems to have a custom device with TTYs whose platform symlink is missing, causing an the "unknown serial device subsystem" branch to be taken erroneously.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.05%. Comparing base (65d0c62) to head (be7dac0).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #54      +/-   ##
==========================================
+ Coverage   89.00%   89.05%   +0.05%     
==========================================
  Files          19       19              
  Lines        2800     2804       +4     
==========================================
+ Hits         2492     2497       +5     
+ Misses        308      307       -1     

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[Copilot]

Pull request overview

This PR fixes a Linux serial port enumeration crash when encountering TTY devices whose sysfs entries lack a resolvable subsystem symlink (as seen on GitHub Actions runner VMs). It hardens linux_list_serial_ports() to skip broken/nonexistent sysfs paths and adds regression coverage to prevent UnboundLocalError from recurring.

Changes:

• Use Path.resolve(strict=True) when resolving tty devices and their subsystem, skipping entries that can’t be resolved.
• Ensure the “unknown subsystem” branch no longer falls through to results.append(info) by explicitly continuing.
• Add Linux tests for devices with no subsystem and for an empty sysfs tree; run serialx.tools.list_ports in CI to surface enumeration failures early.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
serialx/platforms/serial_linux.py	Makes sysfs resolution strict and avoids appending undefined info for unknown subsystems.
tests/test_list_serial_ports_linux.py	Adds regression tests for missing-subsystem TTYs and empty listings.
.github/workflows/ci.yml	Updates Python setup to check-latest: true and runs port listing in test jobs to catch runtime enumeration issues.

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