Test integration

Sends a real test message to a candidate URL without saving it: a meta-only document with `meta.valid`, true only when the provider answers 2xx. `{integration_type}` (path, default `slack`) is `slack` or `discord`.\n\n**Body** (JSON): `url` (string, required) - same rules as \"Save integration\"; an empty URL answers 422 rather than a failed test.\n\nA read-only token can read the integration's URL but cannot send a test (403) - sending one requires a full-access token. A configuration of another organization answers 404 whatever the body. Rate limited to 10 test messages per minute per token (shared with the dashboard and the MCP test tool by credential): the 11th call within a minute answers 429 with no message sent. Never run this request against production with a real webhook URL.