fix: accept Optional[str] in _convert_primitive_type

The _convert_primitive_type function already handles None internally (line 186),
so the type hint should accept Optional[str] not just str.

This fixes the ty type checker error:
  error[invalid-argument-type]: Argument to function _convert_primitive_type is incorrect
  Expected `str`, found `str | None`

✅ Verified with: poetry run ty check src/
✅ All tests pass

Author: Doug Borg

src/openapi_python_generator/language_converters/python/model_generator.py

@@ -164,12 +164,12 @@ def _collect_unique_imports(conversions: List[TypeConversion]) -> Optional[List[
 
 
 def _convert_primitive_type(
-    type_str: str, required: bool
+    type_str: Optional[str], required: bool
 ) -> TypeConversion:
     """
     Handle simple primitive type conversion (string, int, float, bool, object, null, Any).
 
-    :param type_str: Normalized type string
+    :param type_str: Normalized type string (can be None for schemas without a type)
     :param required: Whether the field is required
     :return: TypeConversion for the primitive type
     """