Send an MCP JSON-RPC request

Streamable HTTP transport endpoint for the Superset MCP server (also served at the legacy alias /api/v2/agent/mcp). Send initialize, then tools/list to enumerate the available tools, then tools/call to act on the authenticated user's tasks, workspaces, agents, automations, terminals, hosts, and projects. Responses are application/json or text/event-stream depending on the request's Accept header. Rate limit: 600 requests per 60 seconds per credential, reported in RateLimit-* headers; over the limit returns 429 with Retry-After. A plain GET without Accept: text/event-stream returns a JSON description of the server instead of opening a stream.