fix: implement SaveState/LoadState for DecisionTreeAsyncRegressionBase

Resolves review comment on DecisionTreeAsyncRegressionBase.cs:1020
- Implemented SaveState by delegating to Serialize() with proper stream validation
- Implemented LoadState by delegating to Deserialize() with proper stream validation
- Added null checks, stream capability validation, and empty stream validation

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: ooples

src/Regression/DecisionTreeAsyncRegressionBase.cs

@@ -1008,14 +1008,24 @@ public virtual void ApplyGradients(Vector<T> gradients, T learningRate)
     /// </summary>
     public virtual void SaveState(Stream stream)
     {
-        throw new NotImplementedException("SaveState is not yet implemented for AsyncDecisionTreeRegressionBase. Consider serializing the tree structure explicitly.");
+        if (stream == null) throw new ArgumentNullException(nameof(stream));
+        if (!stream.CanWrite) throw new ArgumentException("Stream must be writable.", nameof(stream));
+        var data = Serialize();
+        stream.Write(data, 0, data.Length);
+        stream.Flush();
     }
 
     /// <summary>
     /// Loads the model's state from a stream.
     /// </summary>
     public virtual void LoadState(Stream stream)
     {
-        throw new NotImplementedException("LoadState is not yet implemented for AsyncDecisionTreeRegressionBase. Consider deserializing the tree structure explicitly.");
+        if (stream == null) throw new ArgumentNullException(nameof(stream));
+        if (!stream.CanRead) throw new ArgumentException("Stream must be readable.", nameof(stream));
+        using var ms = new MemoryStream();
+        stream.CopyTo(ms);
+        var data = ms.ToArray();
+        if (data.Length == 0) throw new InvalidOperationException("Stream contains no data.");
+        Deserialize(data);
     }
 }