Reject both object_name and object_names in cross_section.

Codex flagged that 0.1.7 accepted both selectors simultaneously and
silently preferred object_names. In automated tool chains a stale
object_name left alongside an updated object_names would produce a
different slice set with no error, masking incorrect geometry checks.

blender_cross_section and blender_cross_section_gallery now raise
ValueError when both are present, validated at both the server tool
wrapper and the addon handler.

Backwards compatible: passing exactly one selector continues to work.