Subform expressions explicit dataType reference fails with more than one data element

### Description of the bug

https://altinn.slack.com/archives/C02EJ9HKQA3/p1744022306857699

It looks like we try to load the subform data type in `DatamodelsProvider` when the data type is explicitly referenced in an expression (tableColumns or entryDisplayName which uses data from each subform element). If there are more than one data element, we fail to fetch it since we check that there is exactly one data element.

### Steps To Reproduce

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### Additional Information

_No response_

Start in DatamodelsProvider and trace how an explicitly referenced subform data type is loaded for tableColumns or entryDisplayName expressions. Reproduce with more than one data element in the subform; done when the referenced data type loads successfully instead of failing the single-data-element check.

Written by the indexing model from the issue text.