Rename model based variable names to entity (#24198)

## Context
While looking through the client crate, I noticed that some of the old
functions and variables were still using gpui::model name that was
deprecated during the gpui3 transition. This PR renames those instances
of model to entity to be more inline with gpui3.

In addition, I also renamed `model` to `entity` in cases found by the
below search terms given by @someone13574

- model = cx.
- model: Entity
- model: &Entity
- OpenedModelHandle
- model.update
- model.upgrade
- model = .*\.root (regex)
- parent_model
- model = cx.new
- cx.spawn(move |model

Release Notes:

- N/A

crates/assistant2/src/assistant_panel.rs

@@ -442,7 +442,7 @@ impl AssistantPanel {
 
     fn handle_assistant_configuration_event(
         &mut self,
-        _model: &Entity<AssistantConfiguration>,
+        _entity: &Entity<AssistantConfiguration>,
         event: &AssistantConfigurationEvent,
         window: &mut Window,
         cx: &mut Context<Self>,