fix(plugin-import-export): preview table to include all selected columns regardless of populated data (#12985)

### What?

Ensure the export preview table includes all field keys as columns, even
if those fields are not populated in any of the returned documents.

### Why?

Previously, if none of the documents in the preview result had a value
for a given field, that column would be missing entirely from the
preview table.

### How?

- Introduced a `getFlattenedFieldKeys` utility that recursively extracts
all missing flattened field accessors from the collection’s config that
are undefined

- Updates the preview UI logic to build columns from all flattened keys,
not just the first document

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

@@ -143,6 +143,13 @@ export interface User {
   hash?: string | null;
   loginAttempts?: number | null;
   lockUntil?: string | null;
+  sessions?:
+    | {
+        id: string;
+        createdAt?: string | null;
+        expiresAt: string;
+      }[]
+    | null;
   password?: string | null;
 }
 /**
@@ -502,6 +509,13 @@ export interface UsersSelect<T extends boolean = true> {
   hash?: T;
   loginAttempts?: T;
   lockUntil?: T;
+  sessions?:
+    | T
+    | {
+        id?: T;
+        createdAt?: T;
+        expiresAt?: T;
+      };
 }
 /**
  * This interface was referenced by `Config`'s JSON-Schema