Root cause: AsyncAPIClient.request() passes its parameter directly
to httpx.AsyncClient.request(json=data). When callers pass a pre-serialized
JSON string (data=json.dumps(dict)), httpx re-encodes it via json.dumps(),
resulting in a double-encoded JSON string body instead of a JSON object.
This caused ALL POST/PUT requests with string data to fail — Superset received
a JSON string instead of a JSON object, returning GENERIC_BACKEND_ERROR
('dictionary update sequence element #0 has length 1; 2 is required').
Fix: if data is a string, pass it via httpx parameter (raw body);
if it's a dict/list, pass via for automatic encoding.
Affected callers (6 files) now correctly send JSON objects:
- preview_executor.py: chart data requests
- superset_executor.py
- _run_source.py
- _datasets.py: update_dataset
- _datasets_preview.py: compile_dataset_preview
- _dashboards_write.py
Also simplified preview_executor.fetch_sample_rows back to single-strategy
(chart data API only) since the root cause is now fixed.
