500 error on bulk update page when nodes are large; improve handling of AI Alt Text generation

The attached patch updates the bulk update process to prevent 500 errors on sites with a large number of nodes:

• Replaces loading all entities at once with a paged entity query that fetches results in smaller batches.
• Ensures that entities are processed iteratively instead of being held fully in memory.
• Improves memory usage and execution time by avoiding Entity::loadMultiple() on a very large dataset.
• Maintains existing functionality while making the bulk update page stable and scalable for large content sites.

With this change, bulk alt-text updates now complete successfully even on sites with tens of thousands of nodes, without triggering memory exhaustion or timeouts.

Please review.