test: end-to-end harness for reasoning + tool-history replay
Drives the real POST /conversation/[id] route through runMcpFlow,
executeToolCalls, prepareMessagesWithFiles and MongoDB persistence,
scripting only the OpenAI upstream and the MCP tool server. Asserts on
the messages array captured on the next turn's upstream request, which
is the replayed history as a provider receives it.
Adds A1-interrupted and A2-empty-tool to the live provider harness for
the two shapes only a real backend can rule on.
