Function summarize 

pub async fn summarize(
    client: &dyn LlmProvider,
    transcript: &str,
    original_goal: &str,
    max_chars: usize,
    on_progress: Option<&(dyn Fn(usize) + Sync)>,
) -> AgentResult<String>

Summarise a transcript block via the LLM.

• client — the LLM client to call.
• transcript — serialised older conversation (output of serialize_block).
• original_goal — first user message, truncated to avoid blowing the prompt.
• max_chars — target max length for the summary.
• on_progress — optional callback invoked with cumulative character count as the LLM streams in. Useful for showing “generating summary… X chars”.

Returns the summary text, truncated to max_chars if the LLM over-shoots.