Add legacy content compatibility tests (#484)

[Subomiadekunle]

Pull Request Summary

Closes #484

Adds boneset-api/legacyCompatibility.test.js (54 tests) to confirm the original bone content still works alongside the new Scene Editor. No app code was changed.

The tests check that:

• all 6 bonesets and 11 representative items (at least one per boneset) still load their data, images, descriptions, and colored regions
• a saved scene that uses a legacy image reopens, and the image still loads
• legacy content keeps working while scenes exist

Tested: npm test passes 8/8 suites

Issues found (opened separately):

• [Bug] Axis bone shows as a blank entry and its bone parts are missing #487: Axis shows as a blank dropdown entry
• [Bug] 16 bonesets, bones, and bone parts show "Description not available" and no images #488: 16 items show "Description not available"

Screenshots

PR Checklist

• Project builds and runs
• Tests and linters pass
• Any related documentation has been updated, including JSDoc comments or docstrings

Detailed Description

Test scenes are saved to a temporary folder and deleted afterward, so nothing is written into boneset-api/data/scenes. The Axis bug is marked with it.failing, so Jest will flag that test once #487 is fixed.

[mudabs]

Thank you for adding these legacy compatibility tests. We’re closing this PR for now because the compatibility work is being reorganized to align with the current Scene Editor implementation and related data issues.

Since this PR only adds tests and does not change application code, we will revisit the coverage in a follow-up issue with the updated test scope and known issues addressed separately.