UBspConversionTool::AtLeastOneValidConversionTarget

Returns true if there is at least one valid conversion actor selected (an explicitly selected brush, or a volume if those are included).

Notes:

• An explictly selected subtractive brush is valid, as users may want to delete subtractive brushes through the tool.

• Even with an additive brush, a resulting mesh could still end up empty, for instance if a brush was fully inside a subtractive one.

• Does not check if targets have valid geometry for the "convert then combine" path (for mesh boolean operations), as that currently requires doing an actual conversion.