Add support for fallback models

pr_agent/algo/pr_processing.py

@@ -203,8 +203,10 @@ async def retry_with_fallback_models(f: Callable):
     if not isinstance(fallback_models, list):
         fallback_models = [fallback_models]
     all_models = [model] + fallback_models
-    for model in all_models:
+    for i, model in enumerate(all_models):
         try:
             return await f(model)
         except Exception as e:
             logging.warning(f"Failed to generate prediction with {model}: {e}")
+            if i == len(all_models) - 1:  # If it's the last iteration
+                raise  # Re-raise the last exception