Purge check: fail when bd_pressure does not answer, add override

A bd_pressure that does not answer used to be reported as "pressure
skipped" and the purge passed unchecked. It now fails the check, so in
enforce mode the printer pauses (and a resume purge stays paused).
PURGE_CHECK_IGNORE_SENSOR lets purges pass without the sensor until the
print ends; _PRINT_CLEANUP clears it so the next print checks again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

macros/print/print_start_end.cfg

@@ -257,6 +257,9 @@ gcode:
     M221 S100
     BED_MESH_CLEAR
     SET_SKEW CLEAR=1
+    {% if 'gcode_macro _PURGE_CHECK_CFG' in printer %}
+    SET_GCODE_VARIABLE MACRO=_PURGE_CHECK_CFG VARIABLE=ignore_missing_sensor VALUE=False
+    {% endif %}
 
 # Retract/unretract that is remembered across prints and restarts.
 # Klipper's G10 only retracts once until a G11, and nothing sends G11 here,