TEST: Restore index attachment test.

[aminemanai2003]

Restores test_add_indexes using the indexes generated in the sample database.

The test now:

• initializes ExpectedLossWidget with the matching XYZ triangle, premium data, and averaging selection;
• opens IndexInventory, selects the first sample index, and confirms it is attached to the premium-index model;
• closes SQLAlchemy sessions and disposes their engines after inventory reads so the copied SQLite fixture can be removed reliably.

Validation:

• uv run --extra test python faslr/samples/db/generate_sample_db.py
• QT_QPA_PLATFORM=offscreen uv run --extra test python -m pytest faslr/tests/index/test_index.py -q — 3 passed
• git diff --check

The complete local GUI suite could not finish headlessly because existing connection and data tests wait for native file dialogs. GitHub Actions runs the suite under Xvfb.

This implementation and test review were completed with AI assistance.

Closes #104.

[aminemanai2003]

Hi @genedan — I’ve restored test_add_indexes in PR #115 using the sample database fixture and reopened it for review. The focused index tests pass locally. I’d appreciate your review when you have time, and I’m happy to adjust the approach if needed.