ifcedit: fix Optional[entity_instance] coercion crash on native JSON values

coerce_value assumed value_str was always a CLI string, but ifcmcp
passes JSON-decoded native types (int, None) straight through. Guard
the Union/Optional "none" check so it only calls .lower() on strings,
and handle native None explicitly.

src/ifcedit/ifcedit/coerce.py

@@ -60,9 +60,11 @@ def coerce_value(
     # Union / Optional
     if origin is typing.Union:
         non_none_types = [a for a in args if a is not type(None)]
-        if value_str.lower() == "none":
+        if isinstance(value_str, str) and value_str.lower() == "none":
             if type(None) in args:
                 return None
+        if value_str is None and type(None) in args:
+            return None
         # Try each non-None type in order
         for t in non_none_types:
             try:

src/ifcedit/tests/test_coerce.py

@@ -57,6 +57,16 @@ class TestOptionalCoercion:
     def test_optional_int(self):
         assert coerce_value("42", Optional[int]) == 42
 
+    def test_optional_entity_native_int(self, model):
+        # MCP callers pass JSON-decoded native types (int), not CLI strings.
+        wall = model.by_type("IfcWall")[0]
+        result = coerce_value(wall.id(), Optional[ifcopenshell.entity_instance], model)
+        assert result == wall
+
+    def test_optional_entity_native_none(self, model):
+        # JSON null decodes to Python None, not the string "none".
+        assert coerce_value(None, Optional[ifcopenshell.entity_instance], model) is None
+
 
 class TestUnionCoercion:
     def test_union_str_int(self):