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/assistant_tools/src/fetch_tool.rs

@@ -4,7 +4,7 @@ use std::sync::Arc;
 
 use crate::schema::json_schema_for;
 use anyhow::{Context as _, Result, anyhow, bail};
-use assistant_tool::{ActionLog, Tool};
+use assistant_tool::{ActionLog, Tool, ToolResult};
 use futures::AsyncReadExt as _;
 use gpui::{App, AppContext as _, Entity, Task};
 use html_to_markdown::{TagHandler, convert_html_to_markdown, markdown};
@@ -146,10 +146,10 @@ impl Tool for FetchTool {
         _project: Entity<Project>,
         _action_log: Entity<ActionLog>,
         cx: &mut App,
-    ) -> Task<Result<String>> {
+    ) -> ToolResult {
         let input = match serde_json::from_value::<FetchToolInput>(input) {
             Ok(input) => input,
-            Err(err) => return Task::ready(Err(anyhow!(err))),
+            Err(err) => return Task::ready(Err(anyhow!(err))).into(),
         };
 
         let text = cx.background_spawn({
@@ -158,13 +158,15 @@ impl Tool for FetchTool {
             async move { Self::build_message(http_client, &url).await }
         });
 
-        cx.foreground_executor().spawn(async move {
-            let text = text.await?;
-            if text.trim().is_empty() {
-                bail!("no textual content found");
-            }
+        cx.foreground_executor()
+            .spawn(async move {
+                let text = text.await?;
+                if text.trim().is_empty() {
+                    bail!("no textual content found");
+                }
 
-            Ok(text)
-        })
+                Ok(text)
+            })
+            .into()
     }
 }