Add initial implementation of evaluating changes generated by the assistant (#26799)

Release Notes:

- N/A

---------

Co-authored-by: Richard Feldman <oss@rtfeldman.com>
Co-authored-by: Thomas <thomas@zed.dev>

crates/assistant2/src/tool_use.rs

@@ -182,6 +182,13 @@ impl ToolUseState {
             .map_or(false, |results| !results.is_empty())
     }
 
+    pub fn tool_result(
+        &self,
+        tool_use_id: &LanguageModelToolUseId,
+    ) -> Option<&LanguageModelToolResult> {
+        self.tool_results.get(tool_use_id)
+    }
+
     pub fn request_tool_use(
         &mut self,
         assistant_message_id: MessageId,