fix(plugin-import-export): csv export & preview showing full documents for hasMany monomorphic relationships instead of just ID (#13465)

### What?

Fixes an issue where CSV exports and the preview table displayed all
fields of documents in hasMany monomorphic relationships instead of only
their IDs.

### Why?

This caused cluttered output and inconsistent CSV formats, since only
IDs should be exported for hasMany monomorphic relationships.

### How?

Added explicit `toCSV` handling for all relationship types in
`getCustomFieldFunctions`, updated `flattenObject` to delegate to these
handlers, and adjusted `getFlattenedFieldKeys` to generate the correct
headers.

test/plugin-import-export/collections/Pages.ts

@@ -221,6 +221,12 @@ export const Pages: CollectionConfig = {
       relationTo: ['users', 'posts'],
       hasMany: true,
     },
+    {
+      name: 'hasManyMonomorphic',
+      type: 'relationship',
+      relationTo: 'posts',
+      hasMany: true,
+    },
     {
       type: 'collapsible',
       label: 'Collapsible Field',

test/plugin-import-export/int.spec.ts

@@ -598,6 +598,35 @@ describe('@payloadcms/plugin-import-export', () => {
       expect(data[0].hasManyPolymorphic_1_relationTo).toBe('posts')
     })
 
+    it('should export hasMany monomorphic relationship fields to CSV', async () => {
+      const doc = await payload.create({
+        collection: 'exports',
+        user,
+        data: {
+          collectionSlug: 'pages',
+          fields: ['id', 'hasManyMonomorphic'],
+          format: 'csv',
+          where: {
+            title: { contains: 'Monomorphic' },
+          },
+        },
+      })
+
+      const exportDoc = await payload.findByID({
+        collection: 'exports',
+        id: doc.id,
+      })
+
+      expect(exportDoc.filename).toBeDefined()
+      const expectedPath = path.join(dirname, './uploads', exportDoc.filename as string)
+      const data = await readCSV(expectedPath)
+
+      // hasManyMonomorphic
+      expect(data[0].hasManyMonomorphic_0_id).toBeDefined()
+      expect(data[0].hasManyMonomorphic_0_relationTo).toBeUndefined()
+      expect(data[0].hasManyMonomorphic_0_title).toBeUndefined()
+    })
+
     // disabled so we don't always run a massive test
     it.skip('should create a file from a large set of collection documents', async () => {
       const allPromises = []

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

@@ -242,6 +242,7 @@ export interface Page {
           }
       )[]
     | null;
+  hasManyMonomorphic?: (string | Post)[] | null;
   textFieldInCollapsible?: string | null;
   updatedAt: string;
   createdAt: string;
@@ -580,6 +581,7 @@ export interface PagesSelect<T extends boolean = true> {
   excerpt?: T;
   hasOnePolymorphic?: T;
   hasManyPolymorphic?: T;
+  hasManyMonomorphic?: T;
   textFieldInCollapsible?: T;
   updatedAt?: T;
   createdAt?: T;

test/plugin-import-export/seed/index.ts

@@ -159,6 +159,16 @@ export const seed = async (payload: Payload): Promise<boolean> => {
       })
     }
 
+    for (let i = 0; i < 2; i++) {
+      await payload.create({
+        collection: 'pages',
+        data: {
+          title: `Monomorphic ${i}`,
+          hasManyMonomorphic: [posts[1]?.id ?? ''],
+        },
+      })
+    }
+
     for (let i = 0; i < 5; i++) {
       await payload.create({
         collection: 'pages',