PR-R2: Load viewer representations from extras

[cpignedoli]

What

PR-R2 in the representation split. This is stacked on top of #768 (feature/encoded-representation-style-ids).

This PR teaches the structure viewer/manager to read viewer representation masks from AiiDA node extras and to clean stale representation rows when switching between structures.

Why

Encoded representation arrays already survive file upload/download, but stored AiiDA nodes need an extras-based path because get_ase() does not preserve custom viewer state by itself.

This PR intentionally only reads/restores that state. Writing it is kept for the next PR in the chain.

Chain

• PR-R1: Encode structure viewer representation styles #768, encoded representation style IDs
• PR-R2: this PR, load representation masks from extras and drop stale rows
• PR-R3: persist representation masks back to extras

Implemented with Codex.

Validation

python -m pytest tests/test_viewers.py tests/test_structures.py

Result: 35 passed, 20 warnings.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.35%. Comparing base (e0f78bb) to head (d3b7be4).

Files with missing lines	Patch %	Lines
aiidalab_widgets_base/viewers.py	76.47%	4 Missing ⚠️
aiidalab_widgets_base/structures.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@                             Coverage Diff                              @@
##           feature/encoded-representation-style-ids     #769      +/-   ##
============================================================================
- Coverage                                     85.40%   85.35%   -0.06%     
============================================================================
  Files                                            18       18              
  Lines                                          3379     3393      +14     
============================================================================
+ Hits                                           2886     2896      +10     
- Misses                                          493      497       +4     

Flag	Coverage Δ
python-3.12	85.32% <71.42%> (-0.06%)	⬇️
python-3.9	85.33% <71.42%> (-0.06%)	⬇️

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