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/code_action_tool.rs

@@ -1,5 +1,5 @@
 use anyhow::{Context as _, Result, anyhow};
-use assistant_tool::{ActionLog, Tool};
+use assistant_tool::{ActionLog, Tool, ToolResult};
 use gpui::{App, Entity, Task};
 use language::{self, Anchor, Buffer, ToPointUtf16};
 use language_model::{LanguageModelRequestMessage, LanguageModelToolSchemaFormat};
@@ -141,10 +141,10 @@ impl Tool for CodeActionTool {
         project: Entity<Project>,
         action_log: Entity<ActionLog>,
         cx: &mut App,
-    ) -> Task<Result<String>> {
+    ) -> ToolResult {
         let input = match serde_json::from_value::<CodeActionToolInput>(input) {
             Ok(input) => input,
-            Err(err) => return Task::ready(Err(anyhow!(err))),
+            Err(err) => return Task::ready(Err(anyhow!(err))).into(),
         };
 
         cx.spawn(async move |cx| {
@@ -319,7 +319,7 @@ impl Tool for CodeActionTool {
 
                 Ok(response)
             }
-        })
+        }).into()
     }
 }