Migrate coverage reporting from Coveralls to Codecov

[Copilot]

• Update .github/workflows/ci.yml: simplify coverage clover path and replace Coveralls step with Codecov
• Update README.md: replace Coveralls badge with Codecov badge (Markdown syntax)
• Update composer.json: remove php-coveralls/php-coveralls from require-dev

Original prompt

Migrate from Coveralls to Codecov for coverage reporting, mirroring the changes made in brick/math#109.

Changes required

1. .github/workflows/ci.yml

In the phpunit job, make the following changes:

• In the "Run PHPUnit with coverage" step, change the clover output path from build/logs/clover.xml to clover.xml (and remove the mkdir -p build/logs / mkdir -p mkdir -p build/logs line):

  - name: Run PHPUnit with coverage
    run: vendor/bin/phpunit --coverage-clover clover.xml
    if: ${{ matrix.php-version == env.COVERAGE_PHP_VERSION }}

• Replace the "Upload coverage report to Coveralls" step with:

  - name: Upload coverage to Codecov
    uses: codecov/codecov-action@v5
    env:
      CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
    if: ${{ matrix.php-version == env.COVERAGE_PHP_VERSION }}

  (Remove the old step that runs vendor/bin/php-coveralls with COVERALLS_REPO_TOKEN.)

2. README.md

Replace the Coveralls badge with a Codecov badge. Look for the Coveralls badge in the README.md and replace it with:

[![Coverage](https://codecov.io/github/brick/structured-data/graph/badge.svg)](https://codecov.io/github/brick/structured-data)

3. composer.json

Remove php-coveralls/php-coveralls from the require-dev section.

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[BenMorel]

@copilot Don't use HTML in markdown.

[Copilot]

@copilot Don't use HTML in markdown.

Fixed in 8c95e6b — the Codecov badge now uses standard Markdown syntax ([![Coverage Status](...)](...)) consistent with the other badges.

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️