Fix imported viewer representation arrays

[cpignedoli]

Summary

Fix a structure-viewer crash when an incoming ase.Atoms object already contains an _aiidalab_viewer_representation_* array that is not present in the current viewer representation list.

Root Cause

StructureDataViewer._observe_structure correctly iterated over the incoming representation style_id, but the unknown-representation branch tried to read structure.arrays[self.style_id]. StructureDataViewer has no style_id attribute, so importing/changing to a structure carrying representation arrays raised AttributeError.

This can happen in downstream apps when a structure previously modified by representation controls is reused or when switching between structures after applying custom representations.

Changes

• Use the local style_id when importing an unknown representation array.
• Iterate over a copied list of structure array names when removing stale representation arrays, so deletion during cleanup is safe.
• Add a regression test for assigning a structure with an unknown representation array.

Validation

• python -m pytest tests/test_viewers.py::test_structure_data_viewer_imports_unknown_representation_array tests/test_viewers.py::test_structure_data_viewer_representation
• python -m pytest tests/test_viewers.py

Implemented with Codex.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.18%. Comparing base (42ac05e) to head (ad69daf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #766      +/-   ##
==========================================
+ Coverage   85.03%   85.18%   +0.14%     
==========================================
  Files          18       18              
  Lines        3335     3335              
==========================================
+ Hits         2836     2841       +5     
+ Misses        499      494       -5     

Flag	Coverage Δ
python-3.12	85.15% <100.00%> (+0.20%)	⬆️
python-3.9	85.17% <100.00%> (+0.15%)	⬆️

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