Make altText available in trail data

Make [altText](https://github.com/guardian/dotcom-rendering/blob/5be464d2f2947864016b516932efb359a6b53422/dotcom-rendering/src/components/FetchMoreGalleriesData.importable.tsx#L44) data available in trail data coming from frontend. 

Suggested by Jamie in this [PR comment](https://github.com/guardian/dotcom-rendering/pull/14545#discussion_r2406865748)