Render messages as early as possible to show progress (#11569)

This shows "Researching..." as placeholder text as early as possible so
that the user can see the model is working on reading/researching/etc.

This also adds on an `Option<Value>` to the `render_running` function so
that tools can hopefully render based on partially completed JSON (still
to come).

Release Notes:

- N/A

crates/assistant2/src/assistant2.rs

@@ -16,7 +16,8 @@ use crate::{
 use ::ui::{div, prelude::*, Color, Tooltip, ViewContext};
 use anyhow::{Context, Result};
 use assistant_tooling::{
-    AttachmentRegistry, ProjectContext, ToolFunctionCall, ToolRegistry, UserAttachment,
+    tool_running_placeholder, AttachmentRegistry, ProjectContext, ToolFunctionCall, ToolRegistry,
+    UserAttachment,
 };
 use client::{proto, Client, UserStore};
 use collections::HashMap;
@@ -864,6 +865,10 @@ impl AssistantChat {
                     }
                 }
 
+                if message_elements.is_empty() {
+                    message_elements.push(tool_running_placeholder());
+                }
+
                 div()
                     .when(is_first, |this| this.pt(padding))
                     .child(