assistant2: Decouple scripting tool from the Tool trait (#26382)

This PR decouples the scripting tool from the `Tool` trait while still
allowing it to be used as a tool from the model's perspective.

This will allow us to evolve the scripting tool as more of a first-class
citizen while still retaining the ability to have the model call it as a
regular tool.

Release Notes:

- N/A

crates/assistant2/src/tool_use.rs

@@ -267,6 +267,7 @@ impl ToolUseState {
 pub struct PendingToolUse {
     pub id: LanguageModelToolUseId,
     /// The ID of the Assistant message in which the tool use was requested.
+    #[allow(unused)]
     pub assistant_message_id: MessageId,
     pub name: Arc<str>,
     pub input: serde_json::Value,