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/create_file_tool.rs

@@ -1,6 +1,6 @@
 use crate::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 language_model::LanguageModelRequestMessage;
 use language_model::LanguageModelToolSchemaFormat;
@@ -73,14 +73,16 @@ impl Tool for CreateFileTool {
         project: Entity<Project>,
         action_log: Entity<ActionLog>,
         cx: &mut App,
-    ) -> Task<Result<String>> {
+    ) -> ToolResult {
         let input = match serde_json::from_value::<CreateFileToolInput>(input) {
             Ok(input) => input,
-            Err(err) => return Task::ready(Err(anyhow!(err))),
+            Err(err) => return Task::ready(Err(anyhow!(err))).into(),
         };
         let project_path = match project.read(cx).find_project_path(&input.path, cx) {
             Some(project_path) => project_path,
-            None => return Task::ready(Err(anyhow!("Path to create was outside the project"))),
+            None => {
+                return Task::ready(Err(anyhow!("Path to create was outside the project"))).into();
+            }
         };
         let contents: Arc<str> = input.contents.as_str().into();
         let destination_path: Arc<str> = input.path.as_str().into();
@@ -106,5 +108,6 @@ impl Tool for CreateFileTool {
 
             Ok(format!("Created file {destination_path}"))
         })
+        .into()
     }
 }