fix(plugin-import-export): collectionSlug field regression from hidden field changes (#13917)

### 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

test/plugin-import-export/payload-types.ts

@@ -198,7 +198,7 @@ export interface Page {
               root: {
                 type: string;
                 children: {
-                  type: string;
+                  type: any;
                   version: number;
                   [k: string]: unknown;
                 }[];