Apply the GUI's Mixed Filament Rules on the CLI (#15636)

A valid mixed filament already slices the same on the CLI as in the GUI;
these are the places where the CLI still skipped a rule the GUI applies.

- Keep the prime tower when a mixed filament is used, even if every
  --load-filaments preset is the same. A mixed filament swaps between its
  components every layer, so turning the tower off left the swaps with
  nothing to purge on.
- Leave a mixed slot's row and column of the flush matrix at zero when
  --filament-colour triggers a recompute, as the GUI does; a mixed slot
  never reaches a nozzle.
- Refuse a mixed slot that has no filament of its own. Feature filament
  ids aimed at it were past the filament count, got reset to filament 1
  and the model silently printed in one colour.
- Refuse a plate that uses a mixed filament whose components are
  different filament types, the type half of the GUI's
  Sidebar::has_broken_mixed_filament. Missing or out-of-range components
  are already rejected for the whole project by validate().
  get_extruders_under_cli gains an expand_mixed_slots flag so the gate
  can see mixed slots rather than their components; existing callers
  keep the expanded list.

Both refusals exit with the new CLI_MIXED_FILAMENT_INVALID (-69).

src/slic3r/GUI/PartPlate.cpp

@@ -1717,7 +1717,7 @@ std::vector<int> PartPlate::get_extruders(bool conside_custom_gcode, const Dynam
 	return plate_extruders;
 }
 
-std::vector<int> PartPlate::get_extruders_under_cli(bool conside_custom_gcode, DynamicPrintConfig& full_config) const
+std::vector<int> PartPlate::get_extruders_under_cli(bool conside_custom_gcode, DynamicPrintConfig& full_config, bool expand_mixed_slots) const
 {
     std::vector<int> plate_extruders;
 
@@ -1878,7 +1878,7 @@ std::vector<int> PartPlate::get_extruders_under_cli(bool conside_custom_gcode, D
     // Expand mixed filament slots to their physical components. A mixed slot is virtual and
     // is never loaded into a tray, so callers (AMS mapping, filament checks) must see the
     // physical filaments it resolves to instead.
-    {
+    if (expand_mixed_slots) {
         auto* is_mixed_opt = full_config.option<ConfigOptionBools>("filament_is_mixed");
         auto* comp_strs_opt = full_config.option<ConfigOptionStrings>("filament_mixed_components");
         if (is_mixed_opt && comp_strs_opt && has_any_mixed_filament(is_mixed_opt->values)) {