chore: add codeconv badge

[kdihalas]

This pull request updates the README.md file to add a code coverage badge, making it easier to monitor test coverage status directly from the project documentation.

Documentation improvements:

• Added a Codecov badge to the README.md to display code coverage information alongside the CI badge.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.08%. Comparing base (a64622a) to head (24b0991).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main       #5   +/-   ##
=======================================
  Coverage   84.08%   84.08%           
=======================================
  Files           4        4           
  Lines         245      245           
=======================================
  Hits          206      206           
  Misses         32       32           
  Partials        7        7           

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