Gateway chat-send failure finals can leave Activity sessions running unless handled like thrown agent errors

A webchat gateway used a CLI-backed agent runner. When the runner threw after an agent run started, the chat-send catch path persisted the user turn, appended an assistant error, and marked the Activity/session store entry failed. But when the runner resolved normally with a final failure reply payload after an internal retry, the post-dispatch success path treated the run as ok. The UI could show a run error while the durable Activity entry remained running, sometimes with a missing transcript file.