fix(acp): drain turn updates before completion

[wgqqqqq]

Summary

• Drain pending ACP session updates after StopReason before emitting turn completion
• Replace non-streaming ACP read_to_string usage with local turn reading that drains tail text updates
• Preserve context usage and tool tracking while draining tail updates

Testing

• cargo fmt -p bitfun-acp
• cargo check -p bitfun-acp
• cargo test -p bitfun-acp