assistant2: Rework enabled tool representation (#27527)

This PR reworks how we store enabled tools in the `ToolWorkingSet`.

We now track them based on which tools are explicitly enabled, rather
than by the tools that have been disabled.

Also fixed an issue where switching profiles wouldn't properly set the
right tools.

Release Notes:

- N/A

crates/assistant2/src/profile_selector.rs

@@ -78,7 +78,7 @@ impl ProfileSelector {
 
                             thread_store
                                 .update(cx, |this, cx| {
-                                    this.load_default_profile(cx);
+                                    this.load_profile_by_id(&profile_id, cx);
                                 })
                                 .log_err();
                         }