Relations form - if foreign key field is removed from the form of the child layer, the relation reference is not created

[alex-cit]

Description
Reported in MerginMaps/docs#721
The form for a child layer needs to contain the foreign key field, otherwise the relation reference is not created when adding child features in the mobile app.

Environment
Production

Application (+ app version, build, operating system)
mobile app v2025.8.0, Android 14
QGIS version | 3.44.4-Solothurn

Steps to reproduce

1. Clone the documentation project https://app.merginmaps.com/projects/documentation/forms-relations/tree.
2. In QGIS, remove the fk-uuid from the Attributes Form of the child-amenities layer using the Drag and drop designer
3. Sync the project and open it in the mobile app
4. Open the form of a parent feature (a park) and add a new child feature using park-amenities-relation

Actual results
The child feature is created, but does not contain the relation reference to the parent feature ( the foreign key field fk-uuid is empty).

Expected results
The relation reference should be created even if the field is not in the form - the foreign key fk-uuid should contain the uuid of the parent feature. This is how it works in QGIS.

[tomasMizera]

Most likely related to/duplicate of #1764