assistant2: Restructure storage of tool uses and results (#21194)

This PR restructures the storage of the tool uses and results in
`assistant2` so that they don't live on the individual messages.

It also introduces a `LanguageModelToolUseId` newtype for better type
safety.

Release Notes:

- N/A

crates/assistant2/src/assistant_panel.rs

@@ -102,7 +102,12 @@ impl AssistantPanel {
                         let task = tool.run(tool_use.input, self.workspace.clone(), cx);
 
                         self.thread.update(cx, |thread, cx| {
-                            thread.insert_tool_output(tool_use.id.clone(), task, cx);
+                            thread.insert_tool_output(
+                                tool_use.assistant_message_id,
+                                tool_use.id.clone(),
+                                task,
+                                cx,
+                            );
                         });
                     }
                 }