### What?
Improves the flattening logic used in the import-export plugin to
correctly handle polymorphic relationships (both `hasOne` and `hasMany`)
when generating CSV columns.
### Why?
Previously, `hasMany` polymorphic relationships would flatten their full
`value` object recursively, resulting in unwanted keys like `createdAt`,
`title`, `email`, etc. This change ensures that only the `id` and
`relationTo` fields are included, matching how `hasOne` polymorphic
fields already behave.
### How?
- Updated `flattenObject` to special-case `hasMany` polymorphic
relationships and extract only `relationTo` and `id` per index.
- Refined `getFlattenedFieldKeys` to return correct column keys for
polymorphic fields:
- `hasMany polymorphic → name_0_relationTo`, `name_0_id`
- `hasOne polymorphic → name_relationTo`, `name_id`
- `monomorphic → name` or `name_0`
- **Added try/catch blocks** around `toCSVFunctions` calls in
`flattenObject`, with descriptive error messages including the column
path and input value. This improves debuggability if a custom `toCSV`
function throws.
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