TestingBot IP ranges for firewall whitelisting

Returns the up-to-date list of public IPv4 addresses used by TestingBot test machines. Allow these in your firewall when running tests against staging or production behind corporate VPN/network rules. The response is a flat JSON array of IPv4 strings (no CIDR ranges, no metadata) — re-fetch periodically since the pool changes as we scale capacity. Unauthenticated; no key/secret required.