driver: don't assume target is set in __str__

[Sanjays2402]

Description

Driver.__attrs_post_init__ temporarily sets self.target = None while binding is in progress, so a driver's __str__ must not dereference it. When Target.bind_driver raised BindingError(f"{client} got unexpected bindings: ..."), formatting that message called SerialDriver.__str__ and blew up with AttributeError: 'NoneType' object has no attribute 'name', hiding the real cause. QemuDriver.__str__ had the same pattern, so both were fixed (as suggested in the issue).

Tested locally: the new test_unexpected_binding_reports_binding_error fails with the reported AttributeError without the source change and passes with it; tests/test_serialdriver.py and tests/test_target.py are green (52 passed).

Checklist

• Documentation for the feature
• Tests for the feature
• The arguments and description in doc/configuration.rst have been updated
• Add a section on how to use the feature to doc/usage.rst
• Add a section on how to use the feature to doc/development.rst
• PR has been tested
• Man pages have been regenerated

Fixes #1414

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.0%. Comparing base (ebe3535) to head (baff58b).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/driver/qemudriver.py	0.0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1942     +/-   ##
========================================
- Coverage    61.0%   61.0%   -0.1%     
========================================
  Files         182     182             
  Lines       14881   14883      +2     
========================================
- Hits         9086    9082      -4     
- Misses       5795    5801      +6     

Flag	Coverage Δ
3.10	61.0% <50.0%> (-0.1%)	⬇️
3.11	61.0% <50.0%> (-0.1%)	⬇️
3.12	61.0% <50.0%> (-0.1%)	⬇️
3.13	60.9% <50.0%> (-0.1%)	⬇️
3.14	60.9% <50.0%> (-0.1%)	⬇️

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