Update how project uses Codecov

[ryancheley]

In a blog post by Hynek he says

So, I removed Codecov from all my projects and it’s glorious! Not only did I get rid of a flaky dependency, it also simplified my workflow. The interesting parts are the following: ...

He also posted this github worklow

coverage:
  name: Ensure 100% test coverage
  runs-on: ubuntu-latest
  needs: tests
  if: always()

  steps:
    - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
      with:
        persist-credentials: false
    - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
      with:
        # latest version you support, so it understands all syntax
        python-version: "3.14"
    - uses: hynek/setup-cached-uv@4300ec2180bc77d705e626a34e381b81a4772c51 # v2.5.0

    - name: Download coverage data
      uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
      with:
        pattern: coverage-data-*
        merge-multiple: true

    - name: Combine coverage and fail if it's <100%
      run: |
        uv tool install coverage

        coverage combine
        coverage html --skip-covered --skip-empty

        # Report and write to summary.
        coverage report --format=markdown >> $GITHUB_STEP_SUMMARY

        # Report again and fail if under 100%.
        coverage report --fail-under=100

    - name: Upload HTML report if check failed
      uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
      with:
        name: html-report
        path: htmlcov
      if: ${{ failure() }}

This issue serves as a way to migrate from Codecov to Hynek's suggested one.

[github-actions]

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