fix(router): bind fallback_kwargs before try for each iteration

Greptile: except used fallback_kwargs assigned only inside try, risking
UnboundLocalError or a stale dict from a prior loop iteration. Set
fallback_kwargs = kwargs before each try; first line in try remains
safe_deep_copy(kwargs).

Co-authored-by: Cursor <cursoragent@cursor.com>

litellm/router_utils/fallback_event_handlers.py

@@ -123,6 +123,10 @@ async def run_async_fallback(
     for mg in fallback_model_group:
         if mg == original_model_group:
             continue
+        # Bind before try so except always has a defined dict; reset each
+        # iteration so we never log a stale copy from a prior attempt. Until
+        # safe_deep_copy runs, this aliases the caller kwargs (same as legacy).
+        fallback_kwargs = kwargs
         try:
             # Keep each fallback attempt isolated from mutations made while
             # preparing or sending a previous attempt.