ifcgeom: ignore BboxSubstitutionThreshold in the SWIG wrapper

The new bbox-substitution-threshold setting struct was added to
ConversionSettings.h without the matching %ignore entry that every
other SettingBase derived setting struct has in IfcPython.i. Without
it, SWIG generated a stray BboxSubstitutionThreshold class exposing
just its name/description constants, which is not how any of the
other ~50 setting structs are surfaced (they are all read and written
through the generic Settings.get_/set_ string API instead) and which
made ifcopenshell_wrapper.pyi drift from the compiled wrapper, failing
test_validate_stub.py in CI. Adding the ignore entry, in the same
position as the struct itself was inserted in ConversionSettings.h,
restores parity with its siblings and needs no stub changes.

Generated with the assistance of an AI coding tool.

src/ifcwrap/IfcPython.i

@@ -148,6 +148,7 @@
 %ignore CircleSegments;
 %ignore CgalSmoothAngleDegrees;
 %ignore KeepBoundingBoxes;
+%ignore BboxSubstitutionThreshold;
 %ignore SurfaceColour;
 %ignore ComputeCurvature;
 %ignore FunctionStepType;