Check subdomain availability

Request to check if a subdomain is available.

Attributes: subdomain: The subdomain to check (e.g., 'my-app' for my-app.apps.rebolt.ai).

Response indicating whether a subdomain is available.

Attributes: available: Whether the subdomain is available. subdomain: The subdomain that was checked. full_domain: The full domain that would be created (e.g., my-app.apps.rebolt.ai).