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/assistant_tooling/src/assistant_tooling.rs

@@ -5,5 +5,6 @@ mod tool_registry;
 pub use attachment_registry::{AttachmentRegistry, LanguageModelAttachment, UserAttachment};
 pub use project_context::ProjectContext;
 pub use tool_registry::{
-    LanguageModelTool, ToolFunctionCall, ToolFunctionDefinition, ToolOutput, ToolRegistry,
+    tool_running_placeholder, LanguageModelTool, ToolFunctionCall, ToolFunctionDefinition,
+    ToolOutput, ToolRegistry,
 };