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/metrics.rs

@@ -229,6 +229,7 @@ fn render_metrics(stats: &Stats) -> String {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use http_body_util::BodyExt;
 
     #[test]
     fn test_render_metrics_format() {