### What?
- Fixes collectionSlug field not being populated, breaking export
downloads
- Works around recent UI changes that prevent custom components from
rendering for `admin.hidden` fields
- Removes `admin.hidden: true` - as the component already ensures a
hidden state by returning `null`
### Why?
- This merged [PR](https://github.com/payloadcms/payload/pull/13869)
changed how `admin.hidden` fields are rendered, causing them to bypass
custom field components entirely. The `collectionSlug` field relied on a
custom `CollectionField` component to set its value from the
ImportExportProvider context.
### How?
- Removes `admin.hidden: true`
- Field remains visually hidden with `null` return but custom component
logic now executes properly
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