test: address PR review feedback for blocked-task schema

backend/tests/test_openapi_blocked_task_error_schema.py

@@ -13,4 +13,15 @@ def test_openapi_blocked_task_error_includes_code_field() -> None:
 
     # `code` is optional but must be documented for clients.
     assert "code" in props
-    assert props["code"].get("anyOf")
+    required_fields = blocked_detail.get("required", [])
+    assert "code" not in required_fields
+
+    code_schema = props["code"]
+    any_of = code_schema.get("anyOf")
+    assert isinstance(any_of, list) and any_of
+
+    has_string_branch = any(branch.get("type") == "string" for branch in any_of)
+    assert has_string_branch
+
+    has_null_branch = any(branch.get("type") == "null" or branch.get("nullable") is True for branch in any_of)
+    assert has_null_branch