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/libslic3r/Utils.hpp

@@ -70,6 +70,7 @@
 #define CLI_FILAMENT_CAN_NOT_MAP      -66
 #define CLI_ONLY_ONE_TPU_SUPPORTED      -67
 #define CLI_FILAMENTS_NOT_SUPPORTED_BY_EXTRUDER  -68
+#define CLI_MIXED_FILAMENT_INVALID      -69
 
 #define CLI_SLICING_ERROR                  -100
 #define CLI_GCODE_PATH_CONFLICTS           -101