fix: add #[cfg(test)] to unused ProxyError import

The ProxyError import in tls.rs is only used in test code
(validate_server_hello_structure function), so guard it with
#[cfg(test)] to eliminate the unused import warning.

src/protocol/tls.rs

@@ -7,6 +7,7 @@
 #![allow(dead_code)]
 
 use crate::crypto::{sha256_hmac, SecureRandom};
+#[cfg(test)]
 use crate::error::ProxyError;
 use super::constants::*;
 use std::time::{SystemTime, UNIX_EPOCH};