🔧 Sanity-check SWITCHING_TOOLHEAD_X_POS (#23985)

Marlin/src/inc/SanityCheck.h

 #endif
 
 /**
+ * Generic Switching Toolhead requirements
+ */
+#if ANY(SWITCHING_TOOLHEAD, MAGNETIC_SWITCHING_TOOLHEAD, ELECTROMAGNETIC_SWITCHING_TOOLHEAD)
+  constexpr float thpx[] = SWITCHING_TOOLHEAD_X_POS;
+  static_assert(COUNT(thpx) == EXTRUDERS, "SWITCHING_TOOLHEAD_X_POS must be an array EXTRUDERS long.");
+#endif
+
+/**
  * Switching Toolhead requirements
  */
 #if ENABLED(SWITCHING_TOOLHEAD)

buildroot/tests/BIGTREE_GTR_V1_0

 restore_configs
 opt_set MOTHERBOARD BOARD_BTT_GTR_V1_0 SERIAL_PORT -1 \
         EXTRUDERS 3 TEMP_SENSOR_1 1 TEMP_SENSOR_2 1 \
-        SERVO_DELAY '{ 300, 300, 300 }'
+        SERVO_DELAY '{ 300, 300, 300 }' \
+        SWITCHING_TOOLHEAD_X_POS '{ 215, 0 ,0 }'
 opt_enable SWITCHING_TOOLHEAD TOOL_SENSOR
 exec_test $1 $2 "BigTreeTech GTR | Switching Toolhead | Tool Sensors" "$3"