Function plan_nip42_authentication 

pub fn plan_nip42_authentication(
    identity: &LocalIdentity,
    challenge: &str,
    relay_url: &str,
    created_at: u64,
) -> Result<SignedNostrEvent, SoftchatError>

Create a signed NIP-42 authentication event for one challenge and relay.

Errors

Returns SoftchatError::InvalidRelayAuthentication for invalid context or a signing failure.