feat: Implement LLEN command to get list length

This commit introduces the LLEN command, which allows users to retrieve the length of a list stored in the database.

- Added `HandleLLengthCommand` function to `ListOperations.cs` to handle the LLEN command logic. This function retrieves the list associated with the given key and returns its length. If the key does not exist, it returns 0.
- Updated `Firefly.cs` to include the LLEN command in the command processing logic, mapping the "LLEN" command to the `HandleLLengthCommand` function.

src/Firefly.cs

@@ -189,6 +189,9 @@ namespace Firefly
                     case "LINDEX":
                         return HandleLIndexCommand(parts.Length > 1 ? string.Join(" ", parts.Skip(1)) : "");
 
+                    case "LLEN":
+                        return HandleLLengthCommand(parts.Length > 1 ? parts[1] : "");
+
                     case "LRANGE":
                         return HandleLRangeCommand(parts.Length > 1 ? string.Join(" ", parts.Skip(1)) : "");
 

src/ListOperations.cs

@@ -152,6 +152,30 @@ namespace Firefly
                 : Encoding.UTF8.GetBytes("$-1\r\n");
         }
 
+        /// <summary>
+        /// Handles the LLEN command which returns the length of a list.
+        /// </summary>
+        /// <param name="args">Command arguments containing the key</param>
+        /// <returns>The length of the list or 0 if the key does not exist</returns>
+        static byte[] HandleLLengthCommand(string args)
+        {
+            if (string.IsNullOrWhiteSpace(args))
+            {
+                return Encoding.UTF8.GetBytes("-ERR wrong number of arguments for 'llen' command\r\n");
+            }
+
+            string key = args.Trim();
+            int length = 0;
+
+            // Use helper function to safely read the list with read lock
+            ListStoreWithReadLock(key, list =>
+            {
+                length = list.Count;
+            });
+
+            return Encoding.UTF8.GetBytes($":{length}\r\n");
+        }
+
         /// <summary>
         /// Handles the LRANGE command which returns a range of elements from a list.
         /// </summary>