fix(mllm): bind the clean-store key for every request, not only the finishing one

524d9c3aa initialised _clean_store_tokens inside the finish branch, but the
response is constructed on every path through the loop. An unfinished request
therefore raised UnboundLocalError, the scheduler logged "1 requests failed
permanently", and EVERY turn of a media conversation returned ctx=0 -- a
harder failure than the cache bug the commit was fixing.

Caught by running the live reproduction, not by the suite: the tests exercise
the finish path, where the binding exists.