Test data source connectivity

Probe the configured connector. Postgres performs `SELECT 1`; OpenFinance fetches a small accounts page; Openi requests page 1 of invoices. Always returns 200; success is reflected in `ok`.

curl -X POST "https://example.com/v1/data-sources/497f6eca-6276-4993-bfeb-53cbbbba6f08/test"

{  "ok": true,  "status": 0,  "attempts": 0,  "sampleCount": 0,  "error": "string"}