acp: Cancel editing when focus is lost and message was not changed (#36822)

Release Notes:

- N/A

crates/acp_thread/src/acp_thread.rs

@@ -509,7 +509,7 @@ impl ContentBlock {
         "`Image`".into()
     }
 
-    fn to_markdown<'a>(&'a self, cx: &'a App) -> &'a str {
+    pub fn to_markdown<'a>(&'a self, cx: &'a App) -> &'a str {
         match self {
             ContentBlock::Empty => "",
             ContentBlock::Markdown { markdown } => markdown.read(cx).source(),