fix: add missing imports in test code

- Add ProxyError import and fix Result type annotation in tls.rs
- Add Arc import in stats/mod.rs test module
- Add BodyExt import in metrics.rs test module

These imports were missing causing compilation failures in
cargo test --release with 10 errors.

src/stats/mod.rs

@@ -497,6 +497,7 @@ impl ReplayStats {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::sync::Arc;
     
     #[test]
     fn test_stats_shared_counters() {