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/read_file_tool.rs

@@ -2,7 +2,7 @@ use std::sync::Arc;
 
 use crate::{code_symbols_tool::file_outline, schema::json_schema_for};
 use anyhow::{Result, anyhow};
-use assistant_tool::{ActionLog, Tool};
+use assistant_tool::{ActionLog, Tool, ToolResult};
 use gpui::{App, Entity, Task};
 use itertools::Itertools;
 use language_model::{LanguageModelRequestMessage, LanguageModelToolSchemaFormat};
@@ -88,14 +88,14 @@ impl Tool for ReadFileTool {
         project: Entity<Project>,
         action_log: Entity<ActionLog>,
         cx: &mut App,
-    ) -> Task<Result<String>> {
+    ) -> ToolResult {
         let input = match serde_json::from_value::<ReadFileToolInput>(input) {
             Ok(input) => input,
-            Err(err) => return Task::ready(Err(anyhow!(err))),
+            Err(err) => return Task::ready(Err(anyhow!(err))).into(),
         };
 
         let Some(project_path) = project.read(cx).find_project_path(&input.path, cx) else {
-            return Task::ready(Err(anyhow!("Path {} not found in project", &input.path,)));
+            return Task::ready(Err(anyhow!("Path {} not found in project", &input.path,))).into();
         };
 
         let file_path = input.path.clone();
@@ -146,6 +146,6 @@ impl Tool for ReadFileTool {
                     Ok(format!("This file was too big to read all at once. Here is an outline of its symbols:\n\n{outline}\n\nUsing the line numbers in this outline, you can call this tool again while specifying the start_line and end_line fields to see the implementations of symbols in the outline."))
                 }
             }
-        })
+        }).into()
     }
 }