refactor(structure_tree): consolidate StructureTreeModel into data_models and remove duplicate widget implementation

[Arbaaz123676]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other (refactor)

Why is this PR needed?

The structure tree model (StructureTreeModel and StructureTreeItem) was previously defined in both data_models/structure_tree_model.py and widgets/structure_view.py. This duplication increased maintenance
overhead and risk of inconsistent behaviour.

What does this PR do?

• Removes duplicate model implementation from structure_view.py
• Uses the canonical implementation from data_models/structure_tree_model.py
• Updates imports to ensure a single source of truth for the structure tree model

References

Fixes: #232

How has this PR been tested?

• Tested locally using napari
• Verified atlas loads successfully
• Structure tree renders, expands, and selection works as expected
• Confirmed no runtime errors after refactor

Is this a breaking change?

No. This refactor does not change public APIs or user-facing behaviour.

Does this PR require an update to the documentation?

No documentation changes are required as functionality remains unchanged.

Checklist:

• The code has been tested locally
• The code has been formatted with pre-commit

[Arbaaz123676]

@IgorTatarnikov I have consolidated StructureTreeModel into data_models/structure_tree_model.py and removed the duplicate from structure_view.py, with all changes validated locally in napari. I’m happy to make additional changes required.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.02%. Comparing base (4cea26a) to head (58f7740).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #236      +/-   ##
==========================================
+ Coverage   97.03%   98.02%   +0.99%     
==========================================
  Files          14       14              
  Lines         640      556      -84     
==========================================
- Hits          621      545      -76     
+ Misses         19       11       -8     

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