Function compact 

pub async fn compact(
    preparation: &CompactionPreparation,
    api_key: &str,
    model: &str,
    system_prompt_override: Option<&str>,
    thinking_level: ThinkingLevel,
    model_config: Option<ModelConfig>,
) -> Result<CompactionResult, String>

Execute compaction: send messages to the provider for summarisation and return the result ready to append to the session.

model_config should be the session’s current model configuration. thinking_level controls whether the summarization uses reasoning mode.