Wait until the view has no pending summary

Memory.Settle returns when every line of the chat view is built, so a
subagent that starts gets a view with no pending summary, and the view
holds every message so far. A pending line with no running build belongs
to a failed build, which waits for the next message, so Settle returns
an error in place of a wait that never ends. It also ends with its
context, so an interrupted turn does not wait.

Issue: b1ac698