Fix pool dbinuse counter: Always decrement when connection is returned

Co-authored-by: mariuz <18359+mariuz@users.noreply.github.com>

Author: Copilot

lib/pool.js

@@ -47,8 +47,9 @@ class Pool {
                         else
                             self.pooldb.push(db);
 
-                        if (db.connection._pooled)
-                            self.dbinuse--;
+                        // Always decrement dbinuse when a pooled connection is returned
+                        // This connection was counted in dbinuse when given out (line 33)
+                        self.dbinuse--;
                         self.check();
                     });
                 } else {