When /steer is sent during an API call (model thinking), the steer text
sits in _pending_steer until after the next tool batch — which may never
come if the model returns a final response. In that case the steer is
only delivered as a post-run follow-up, defeating the purpose.
Add a pre-API-call drain at the top of the main loop: before building
api_messages, check _pending_steer and inject into the last tool result
in the messages list. This ensures steers sent during model thinking are
visible on the very next API call.
If no tool result exists yet (first iteration), the steer is restashed
for the post-tool drain to pick up — injecting into a user message would
break role alternation.
Three new tests cover the pre-API-call drain: injection into last tool
result, restash when no tool message exists, and backward scan past
non-tool messages.
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