Function commit_assistant_message 

pub fn commit_assistant_message(
    partial_text: String,
    partial_reasoning: String,
    tool_calls: Vec<ToolCall>,
    thinking_signature: Option<String>,
) -> ChatMessage

Build the committed assistant message from a Generating state’s accumulated content. Safe to call with empty text (the model might have responded with only tool calls). Returns the message plus the thinking signature so the reducer can record it separately for Anthropic round-trip.