Streaming tool calls (#29179)

https://github.com/user-attachments/assets/7854a737-ef83-414c-b397-45122e4f32e8



Release Notes:

- Create file and edit file tools now stream their tool descriptions, so
you can see what they're doing sooner.

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/markdown/src/markdown.rs

@@ -192,6 +192,11 @@ impl Markdown {
         self.parse(cx);
     }
 
+    pub fn replace(&mut self, source: impl Into<SharedString>, cx: &mut Context<Self>) {
+        self.source = source.into();
+        self.parse(cx);
+    }
+
     pub fn reset(&mut self, source: SharedString, cx: &mut Context<Self>) {
         if source == self.source() {
             return;