Function build_rest_auth_headers_at 

pub fn build_rest_auth_headers_at(
    wallet: &str,
    signer: &PrivateKeySigner,
    now_ms: u64,
) -> Result<AuthHeaders, AuthError>

Builds REST auth headers with an injected now_ms reference, suitable for deterministic testing.

Errors

Returns AuthError::SigningFailed when the underlying secp256k1 signer errors.