Document blocked-task 409 error code in OpenAPI + add regression tests

backend/tests/test_openapi_blocked_task_error_schema.py

@@ -0,0 +1,16 @@
+# ruff: noqa: INP001, S101
+
+from __future__ import annotations
+
+from app.main import app
+
+
+def test_openapi_blocked_task_error_includes_code_field() -> None:
+    schema = app.openapi()
+
+    blocked_detail = schema["components"]["schemas"]["BlockedTaskDetail"]
+    props = blocked_detail.get("properties", {})
+
+    # `code` is optional but must be documented for clients.
+    assert "code" in props
+    assert props["code"].get("anyOf")