Add codecov badge to readme

[rly]

Motivation

It's currently missing. I think a changelog entry is not needed.

How to test the behavior?

Preview README

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff check . && codespell from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.19%. Comparing base (bf3294e) to head (30cb8b7).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2137   +/-   ##
=======================================
  Coverage   95.19%   95.19%           
=======================================
  Files          28       28           
  Lines        2851     2851           
  Branches      736      736           
=======================================
  Hits         2714     2714           
  Misses         83       83           
  Partials       54       54           

Flag	Coverage Δ
integration	74.04% <ø> (ø)
unit	85.16% <ø> (ø)

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