fix: serialize dict fields as JSON strings in Superset API payloads

[alionar]

Summary

mcp_superset/client.py sends params, json_metadata, and position_json to Superset as Python dicts. Those fields require JSON-encoded strings, so every chart and dashboard create/update returns HTTP 400 "Not a valid string.".

Fixes

• create_chart / update_chart: json.dumps(params) before sending. The reported bug.
• create_dashboard / update_dashboard: same for json_metadata and position_json.
• add_chart_to_dashboard: Superset returns position_json as a string. The old isinstance(x, dict) check fails for every real dashboard and resets the layout to {}, which would wipe the dashboard on the next add. New code parses, mutates, re-serializes.
• update_dashboard_filters: /api/v1/dashboard/{id}/filters does not exist on 4.x and uses a different shape on 6.x ({deleted, modified, reordered}). I rewrote this through json_metadata via the main dashboard PUT, which works on every version.

Test plan

• Line chart via superset_create_chart. Before: 400. After: new chart id.
• superset_add_chart_to_dashboard twice: second call leaves the first chart in place.
• superset_update_dashboard_filters with one filter_select: filter appears in the UI.

Public MCP signatures unchanged. Serialization sites accept dict, list, or pre-stringified JSON.