A tool card is drawn from tool_started and only its tool_call closes it.
When the user stopped a run while a sub-agent was mid-tool, the sub-agent
was cancelled, the parent dropped its sink, and the matching tool_call was
never sent — the card spun until the page was reloaded.

The sub-agent now reports its in-flight tool from the CancelledError
handler, and both batch paths (sequential and parallel) drain the sink
after cancelling instead of discarding it, so that event reaches the
client ahead of the parent's synthetic "stopped by the user" result.

The webclient settles anything still pending at stream_end,
stream_stopped and error as a safety net for a lost frame or a dying
server, with a visible "interrupted" note rather than a silent close.

The TUI needs nothing: it renders tool_call as a result panel and carries
no pending state, so it gets the fix through the backend alone.