fix: add validation for LabelSmoothingFactor to ensure it's within [0, 1]

Added guard in KnowledgeDistillationOptions.Validate() to ensure
LabelSmoothingFactor is within the valid range of [0, 1].

This prevents invalid configurations where LabelSmoothingFactor could be:
- Negative (which would produce invalid smoothed labels)
- Greater than 1 (which would produce invalid probability distributions)

The check is placed alongside other weight/bound validations (Alpha,
FeatureWeight, AttentionWeight, EMADecay) for consistency.

Throws ArgumentException with clear message and nameof(LabelSmoothingFactor)
for proper error reporting.

Author: claude

src/Models/Options/KnowledgeDistillationOptions.cs

@@ -393,6 +393,8 @@ public void Validate()
             throw new ArgumentException("AttentionWeight must be between 0 and 1", nameof(AttentionWeight));
         if (EMADecay < 0 || EMADecay > 1)
             throw new ArgumentException("EMADecay must be between 0 and 1", nameof(EMADecay));
+        if (LabelSmoothingFactor < 0 || LabelSmoothingFactor > 1)
+            throw new ArgumentException("LabelSmoothingFactor must be between 0 and 1", nameof(LabelSmoothingFactor));
         if (SelfDistillationGenerations < 1)
             throw new ArgumentException("SelfDistillationGenerations must be at least 1", nameof(SelfDistillationGenerations));