PR: Exporting glazed roofs as IfcElementAssembly: Regression in 25.4.3

### Problem Description

## 25.4.0 export behaviour
With the the 25.4.0 exporter we were able to export glazed roofs as IfcElementAssembly entities (one use case being elemented ceilings, i.e. the curtain panels are classified as IfcCovering entities). Revit would export this neatly both for IFC2x3 and IFC4:

## 25.4.3 export behaviour
The 25.4.3 exporter shows a different behaviour.

### IFC2x3
For **IFC2x3** it **doesn't export a usable object**.

In Solibri no entity is visible in the model structure:

In BIM Collab Zoom there is an entity visible in the model structure but it does not show any geometry:

There are mulitple validation errors for this entity ("Number of elements (0) violates aggregate bounds") - possibly the reason why the viewers can't display nay geometry?

### IFC4
For **IFC4** it still exports an IfcElementAssembly, but it is **not composed by the individual panels** anymore, i.e. **not exported as an aggregate**. The geometry is still there (but seemingly defined as the geometric representaton of the assembly itself).

### Revit Version

2025.4.x

### IFC for Revit Addon Version

25.4.0

### Windows Version

11 24H2

Reproduce the glazed-roof export with Revit 2025.4.x and IFC for Revit 25.4.3, comparing IFC2x3 and IFC4 against the described 25.4.0 behavior. Inspect the exporter path for IfcElementAssembly and panel aggregation; done means IFC2x3 contains a valid assembly with its elements and IFC4 preserves the individual-panel aggregate and geometry.

Written by the indexing model from the issue text.