agent: Return ToolResult from run inside Tool (#28763)

This is just a refactor which adds no functionality.
We now return a `ToolResult` from `Tool > run(...)`. For now this just
wraps the output task in a struct. We'll use this to implement custom
rendering of tools, see #28621.

Release Notes:

- N/A

crates/agent/src/thread.rs

@@ -1407,8 +1407,8 @@ impl Thread {
     ) -> Task<()> {
         let tool_name: Arc<str> = tool.name().into();
 
-        let run_tool = if self.tools.read(cx).is_disabled(&tool.source(), &tool_name) {
-            Task::ready(Err(anyhow!("tool is disabled: {tool_name}")))
+        let tool_result = if self.tools.read(cx).is_disabled(&tool.source(), &tool_name) {
+            Task::ready(Err(anyhow!("tool is disabled: {tool_name}"))).into()
         } else {
             tool.run(
                 input,
@@ -1421,7 +1421,7 @@ impl Thread {
 
         cx.spawn({
             async move |thread: WeakEntity<Thread>, cx| {
-                let output = run_tool.await;
+                let output = tool_result.output.await;
 
                 thread
                     .update(cx, |thread, cx| {