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/zed/src/main.rs

@@ -476,7 +476,6 @@ fn main() {
             cx,
         );
         assistant_tools::init(cx);
-        scripting_tool::init(cx);
         repl::init(app_state.fs.clone(), cx);
         extension_host::init(
             extension_host_proxy,