emitChatFinal frees buffers on clean run completion, and the
maintenance timer sweeps abortedRuns after ABORTED_RUN_TTL_MS. But
runs that get stuck (e.g. LLM timeout without triggering clean
lifecycle end) are never aborted and their string buffers persist
indefinitely. This is the direct trigger for the StringAdd_CheckNone
OOM crash reported in the issue.
Add a stale buffer sweep in the maintenance timer that cleans up
buffers, deltaSentAt, and deltaLastBroadcastLen for any run not
updated within ABORTED_RUN_TTL_MS, regardless of abort status.
Closes#51821