fix(D010_01_aux): JSON-serialise stub body (HttpCtrl does not auto-encode dicts)

The test passed ${entity_body} straight to Set Stub Reply, but Load Entity returns a Python dict and HttpCtrl's response writer (http_handler.py::__send_response) only encodes the body when it is already a str. A dict falls through: len(dict) reports the number of keys (wrong Content-Length) and wfile.write(dict) raises silently, so the mock sends headers only.

That made the auxiliary RetrieveEntity merge look broken — the broker received no upstream body to gap-fill from. Verified end-to-end against a real upstream: § 9.3.2 aux gap-fill works (local wins for shared attrs, CSR fills the gap).

Serialise the body to a JSON string before passing to Set Stub Reply so the wire payload is non-empty. Documented as testsuite-doubts.md #76 (closed). Verified: D010_01_aux passes.