Improve test coverage for explore_sigma and nested test_covariate

[google-labs-jules]

Improve test coverage by adding missing test cases for explore_sigma and nested test_covariate in scFates/tests/test_w_plots.py. Verified that tests pass and coverage improves.

---

PR created automatically by Jules for task 3647550202235486205 started by @LouisFaure

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.02%. Comparing base (d551d72) to head (3cec252).
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   78.82%   81.02%   +2.19%     
==========================================
  Files          40       40              
  Lines        3977     3979       +2     
  Branches      494      494              
==========================================
+ Hits         3135     3224      +89     
+ Misses        588      503      -85     
+ Partials      254      252       -2     

Files with missing lines	Coverage Δ
scFates/tests/test_w_plots.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.