fix(rustproxy): Use cooperative cancellation for background tasks, prune stale caches and metric entries, and switch tests to dynamic port allocation to avoid port conflicts

rust/crates/rustproxy-http/src/upstream_selector.rs

@@ -131,6 +131,14 @@ impl UpstreamSelector {
         }
     }
 
+    /// Clear stale round-robin counters on route update.
+    /// Resetting is harmless — counters just restart cycling from index 0.
+    pub fn reset_round_robin(&self) {
+        if let Ok(mut counters) = self.round_robin.lock() {
+            counters.clear();
+        }
+    }
+
     fn ip_hash(addr: &SocketAddr) -> usize {
         let ip_str = addr.ip().to_string();
         let mut hash: usize = 5381;