Sanity-check for CASE_LIGHT_PIN (#14518)

Marlin/src/inc/SanityCheck.h

 #endif
 
 /**
- * Test case light not using the same pin as the fan
+ * Case Light requirements
  */
-#if ENABLED(CASE_LIGHT_ENABLE) && CASE_LIGHT_PIN == FAN_PIN
-  #error "You cannot set CASE_LIGHT_PIN equal to FAN_PIN."
+#if ENABLED(CASE_LIGHT_ENABLE)
+  #if !PIN_EXISTS(CASE_LIGHT)
+    #error "CASE_LIGHT_ENABLE requires CASE_LIGHT_PIN to be defined."
+  #elif CASE_LIGHT_PIN == FAN_PIN
+    #error "CASE_LIGHT_PIN conflicts with FAN_PIN. Resolve before continuing."
+  #endif
 #endif
 
 /**

buildroot/share/tests/megaatmega2560-tests

            FWRETRACT ARC_P_CIRCLES ADVANCED_PAUSE_FEATURE CNC_WORKSPACE_PLANES CNC_COORDINATE_SYSTEMS \
            PSU_CONTROL AUTO_POWER_CONTROL POWER_LOSS_RECOVERY POWER_LOSS_PIN POWER_LOSS_STATE \
            LCD_PROGRESS_BAR LCD_PROGRESS_BAR_TEST PINS_DEBUGGING \
-           MAX7219_DEBUG LED_CONTROL_MENU CASE_LIGHT_ENABLE CASE_LIGHT_USE_NEOPIXEL CODEPENDENT_XY_HOMING BACKLASH_COMPENSATION BACKLASH_GCODE
+           MAX7219_DEBUG LED_CONTROL_MENU CASE_LIGHT_MENU CASE_LIGHT_ENABLE CASE_LIGHT_USE_NEOPIXEL CODEPENDENT_XY_HOMING BACKLASH_COMPENSATION BACKLASH_GCODE
 opt_enable SLOW_PWM_HEATERS THERMAL_PROTECTION_CHAMBER
 opt_set TEMP_SENSOR_CHAMBER 3
 opt_set HEATER_CHAMBER_PIN 45