Internal: Fix Composer issue inside Github workflows [TMZ-997]

[hein-obox]

✨ PR Description

Purpose: Fix Composer platform configuration in GitHub workflows by pinning PHP version and restoring testing dependencies to ensure consistent CI/CD execution.

Main changes:

• Added platform PHP version constraint (7.4.33) to prevent Composer dependency resolution issues in CI
• Restored yoast/phpunit-polyfills dependency and added thor-juhasz/phpunit-coverage-check for enhanced test coverage tracking
• Implemented new coverage scripts with 20% threshold checking and phpdbg-based coverage report generation

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