Fix Codecov code coverage analysis from test data

[tleonhardt]

This PR does a few things in pursuit of fixing our integrated code coverage analysis provided by Codecov:

1. It removes the last usage of invoke from our GitHub Actions CI scripts. Now we rely purely on uv for everything.
2. Uploading of data to Codecov is now done using a private token which is stored in GitHub secrets for this repo.
3. Both test data and code coverage are now uploaded.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.15%. Comparing base (ff46021) to head (fad24b5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1434      +/-   ##
==========================================
+ Coverage   97.72%   98.15%   +0.42%     
==========================================
  Files          22       41      +19     
  Lines        5753    13744    +7991     
==========================================
+ Hits         5622    13490    +7868     
- Misses        131      254     +123     

Flag	Coverage Δ
unittests	98.15% <ø> (?)

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