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/now_tool.rs

@@ -2,7 +2,7 @@ use std::sync::Arc;
 
 use crate::schema::json_schema_for;
 use anyhow::{Result, anyhow};
-use assistant_tool::{ActionLog, Tool};
+use assistant_tool::{ActionLog, Tool, ToolResult};
 use chrono::{Local, Utc};
 use gpui::{App, Entity, Task};
 use language_model::{LanguageModelRequestMessage, LanguageModelToolSchemaFormat};
@@ -60,10 +60,10 @@ impl Tool for NowTool {
         _project: Entity<Project>,
         _action_log: Entity<ActionLog>,
         _cx: &mut App,
-    ) -> Task<Result<String>> {
+    ) -> ToolResult {
         let input: NowToolInput = match serde_json::from_value(input) {
             Ok(input) => input,
-            Err(err) => return Task::ready(Err(anyhow!(err))),
+            Err(err) => return Task::ready(Err(anyhow!(err))).into(),
         };
 
         let now = match input.timezone {
@@ -72,6 +72,6 @@ impl Tool for NowTool {
         };
         let text = format!("The current datetime is {now}.");
 
-        Task::ready(Ok(text))
+        Task::ready(Ok(text)).into()
     }
 }