fix(gateway): fix nat mapping resolution mismatch during cache eviction

During async cache eviction by moka, mapping temporary contains orphaned entries. Changing Nat::find() to gracefully reconstruct the SessionKey from mapping resolves lookup misses bridging the eviction gap interval.

Author: yinheli

src/gateway/nat.rs

@@ -123,10 +123,20 @@ impl Nat {
     }
 
     pub async fn find(&self, nat_port: u16) -> Option<Session> {
-        if let Some(addr_key) = self.get_addr_key_by_port_fast(&nat_port).await
-            && let Some(session) = self.cache.get(&addr_key).await
-        {
-            return Some(*session);
+        if let Some(addr_key) = self.get_addr_key_by_port_fast(&nat_port).await {
+            if let Some(session) = self.cache.get(&addr_key).await {
+                return Some(*session);
+            }
+            
+            // Reconstruct session from the mapping if it was evicted from cache
+            // but the eviction listener hasn't removed it from the mapping yet.
+            return Some(Session {
+                src_addr: addr_key.src_addr,
+                dst_addr: addr_key.dst_addr,
+                src_port: addr_key.src_port,
+                dst_port: addr_key.dst_port,
+                nat_port,
+            });
         }
 
         None