feat: centralize OpenAPI validation workflow

[vitormattos]

Summary

Continue #15 by centralizing the OpenAPI validation workflow behind a reusable implementation with a thin catalog caller.

The caller is pinned to immutable commit bfd8f06a82ff2fdd95becc1f752363c640f314ca.

LibreCode adaptations

Compared with the current Nextcloud template:

• remove the Nextcloud-specific github.repository_owner != 'nextcloud-gmbh' condition;
• update checkout, version-matrix, setup-php, setup-node and file-existence action pins;
• use ramsey/composer-install for Composer dependency installation;
• avoid globally installing an npm range controlled by package.json;
• use the npm bundled with the selected Node runtime;
• simplify the generated-file drift check without nested bash -c;
• use public GitHub-hosted runners.

After merge, this can be validated in LibreCodeCoop/extract.