### 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.
3e65111bc1