Fix class property lookup for inverse restrictions

[vibhor-aggr]

Fixes #32.

ThingClass.get_class_properties() and ThingClass.INDIRECT_get_class_properties() assumed every restriction property exposes _class_property_some and _class_property_only. Inverse property constructs do not expose those flags, so classes with inverse restrictions raised AttributeError when their class properties were inspected.

This change uses defensive class-property flag lookup for restriction properties and adds a regression test covering direct and inherited inverse restrictions.

Validation:

uv run --with-editable . python -m unittest test.regtest.Test.test_inverse_1 test.regtest.Test.test_inverse_2 test.regtest.Test.test_inverse_2_get_class_properties test.regtest.Test.test_inverse_3 test.regtest.Test.test_class_prop_12 test.regtest.Test.test_class_prop_13 test.regtest.Test.test_class_prop_14