Attachment store for assistant2 (#11327)

This sets up a way for the user (or Zed) to _push_ context instead of
having the model retrieve it with a function. Our first use is the
contents of the current file.

<img width="399" alt="image"
src="https://github.com/zed-industries/zed/assets/836375/198429a5-82af-4b82-86f6-cb961f10de5c">

<img width="393" alt="image"
src="https://github.com/zed-industries/zed/assets/836375/cfb52444-723b-4fc1-bddc-57e1810c512b">

I heard the asst2 example was deleted in another branch so I deleted
that here too since we wanted the workspace access.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>

crates/assistant2/src/ui/chat_message.rs

@@ -16,6 +16,7 @@ pub struct ChatMessage {
     id: MessageId,
     player: UserOrAssistant,
     message: Option<AnyElement>,
+    tools_used: Option<AnyElement>,
     collapsed: bool,
     on_collapse_handle_click: Box<dyn Fn(&ClickEvent, &mut WindowContext) + 'static>,
 }
@@ -25,6 +26,7 @@ impl ChatMessage {
         id: MessageId,
         player: UserOrAssistant,
         message: Option<AnyElement>,
+        tools_used: Option<AnyElement>,
         collapsed: bool,
         on_collapse_handle_click: Box<dyn Fn(&ClickEvent, &mut WindowContext) + 'static>,
     ) -> Self {
@@ -32,6 +34,7 @@ impl ChatMessage {
             id,
             player,
             message,
+            tools_used,
             collapsed,
             on_collapse_handle_click,
         }
@@ -66,6 +69,10 @@ impl RenderOnce for ChatMessage {
         // Clamp the message height to exactly 1.5 lines when collapsed.
         let collapsed_height = content_padding.to_pixels(cx.rem_size()) + cx.line_height() * 1.5;
 
+        let tools_used = self
+            .tools_used
+            .map(|attachment| div().mt_3().child(attachment));
+
         let content = self.message.map(|message| {
             div()
                 .overflow_hidden()
@@ -75,6 +82,7 @@ impl RenderOnce for ChatMessage {
                 .when(self.collapsed, |this| this.h(collapsed_height))
                 .bg(cx.theme().colors().surface_background)
                 .child(message)
+                .children(tools_used)
         });
 
         v_flex()