Conditional (when helper) gives back Eloquent Builder instance inside the closure instead of the relationship instance

### Laravel Version

11.28.1

### PHP Version

8.3

### Database Driver & Version

_No response_

### Description

Inside the when closure you will not get a relationship instance, so any specific relationship function will not be available.

*Expected:*
$query to be BelongsToMany instance

*Actual:*
$query is a Illuminate\Database\Eloquent\Builder instance

Ex. BelongsToMany relationships, here you will not be able to call the wherePivot*-methods.

```
return MyModel::find(2)
->someRelationship()
->when(true, function($query) {
logger($query::class); // Illuminate\Database\Eloquent\Builder

$query->wherePivotBetween('updated_at', ['2000-05-05', '2024-05-05']);
return $query;
})->get();
```

### Steps To Reproduce

Create a model any kind of relationship and and fire a query that uses the relationship. Like in the example above.

Start by reproducing the relationship query shown in the issue, focusing on the when helper and the BelongsToMany relationship. Trace which object is passed into the closure and add a regression test covering wherePivotBetween; done means the closure receives the relationship instance and its relationship-specific methods work.

Written by the indexing model from the issue text.