Share the request chain of a turn

The request chain moves out of the turn loop into Chain: it sends a
request, logs the replies, runs the tool calls, and sends their results
back until a response has no tool call. A turn builds a chain with the
prompt, the tools, and the input of the turn, so its requests do not
change. The chain also returns the replies of the final answer, because
a subagent needs that text for its report and runs the same chain with
its own prompt, toolbox, and log.

Issue: b1ac698