When the agent is already streaming (system-triggered turn, e.g. autonomous
dispatch at startup) and the user sends a message without an explicit
streamingBehavior, default to followUp instead of steer.
Steer injects mid-stream into the current turn. FollowUp queues the
message as a clean new turn after the system work finishes — which is
what the user expects when they type their first message at startup.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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