🐛 Fix var_index validation

[sunnyosun]

• Fixes Consider negative examples for curate #1788
• var_index is no longer validated upon Curate(), but with curate.validate()
• the using_key param is tested in cellxgene-lamin

[codecov]

Codecov Report

Attention: Patch coverage is 81.73077% with 19 lines in your changes missing coverage. Please review.

Project coverage is 92.01%. Comparing base (9a85e27) to head (06637dd).
Report is 6 commits behind head on main.

Files	Patch %	Lines
lamindb/_curate.py	80.64%	18 Missing ⚠️
lamindb/core/_feature_manager.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1790      +/-   ##
==========================================
- Coverage   92.19%   92.01%   -0.18%     
==========================================
  Files          51       51              
  Lines        5598     5687      +89     
==========================================
+ Hits         5161     5233      +72     
- Misses        437      454      +17     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[github-actions]

🚀 Deployed on https://66b241559bd1ab0c54f66f07--lamindb-qnwk.netlify.app