Fix: Handle MistralCommonTokenizer pad_token setter

MistralCommonTokenizer (used by Devstral) doesn't allow setting
pad_token property. Wrap in try/except to handle this gracefully.

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

backend/model_service.py

@@ -227,7 +227,11 @@ class ModelManager:
             
             # Load tokenizer
             self.tokenizer = AutoTokenizer.from_pretrained(self.model_name)
-            self.tokenizer.pad_token = self.tokenizer.eos_token
+            # Set pad_token if the tokenizer allows it (some like MistralCommonTokenizer don't)
+            try:
+                self.tokenizer.pad_token = self.tokenizer.eos_token
+            except AttributeError:
+                logger.info(f"Tokenizer doesn't support setting pad_token (using default)")
 
             # For Devstral, also load MistralTokenizer for correct encoding
             self.mistral_tokenizer = None