ENH: Disable Uniformat classification if Assembly Code parameter is used for other classification system

### Enhancement Description

## Context
The Uniformat classification system isn't relevant in all markets - because they have their own classification systems. But the Revit user interface for assigning the Assembly Code parameter is really useful and it therefore makes sense to use it for whichever classification system is required for a given project. And yes, we are aware we could use the ClassificationCode shared parameter option and the Standardized Data Tool (aka Classification Manager). But using the Assembly Code parameter with a custom classification text file is - in our opinion - the best choice in terms of UX and ease of deployment.

## The problem
If we use the Assembly Code parameter as our classification parameter in our export settings, like so:


it kinda works, but not entirely:


## Proposed change
If the Assembly Code is picked as a classification parameter in the IFC export settings:
- Do **not** export a Uniformat classification (also asked for in #486)
- Use the Assembly Code Description parameter as the classification **name**


If not, assume it's Uniformat and carry on as usual.

### Revit Version

2025.4.x

### IFC for Revit Addon Version

25.4.0

### Windows Version

11 24H2

The issue names IFC export settings and the Assembly Code and Assembly Code Description parameters, but no source files or tests. Start by locating the export-settings handling and reproduce an export with Assembly Code selected; done means no Uniformat classification is emitted, the Assembly Code Description supplies the classification name, and the default path remains Uniformat.

Written by the indexing model from the issue text.