Function embed_thinking_content 

pub fn embed_thinking_content(
    thinking: Option<&str>,
    content: &str,
    duration_seconds: Option<f64>,
) -> String

Embed thinking content into a message using canonical <think> tags.

Round-trips with parse_thinking_content.

use gglib_core::domain::thinking::embed_thinking_content;

let msg = embed_thinking_content(Some("step 1"), "Answer", Some(3.5));
assert_eq!(msg, "<think duration=\"3.5\">step 1</think>\nAnswer");