assistant panel: Fix panel not reloading after entering credentials (#15531)

This is the revised version of #15527.

We also added new events to notify subscribers when new providers are
added or removed.

Co-Authored-by: Thorsten <thorsten@zed.dev>

Release Notes:

- N/A

---------

Co-authored-by: Thorsten <thorsten@zed.dev>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

crates/language_model/src/provider/google.rs

@@ -44,7 +44,7 @@ pub struct GoogleLanguageModelProvider {
     state: gpui::Model<State>,
 }
 
-struct State {
+pub struct State {
     api_key: Option<String>,
     _subscription: Subscription,
 }
@@ -63,10 +63,10 @@ impl GoogleLanguageModelProvider {
 }
 
 impl LanguageModelProviderState for GoogleLanguageModelProvider {
-    fn subscribe<T: 'static>(&self, cx: &mut gpui::ModelContext<T>) -> Option<gpui::Subscription> {
-        Some(cx.observe(&self.state, |_, _, cx| {
-            cx.notify();
-        }))
+    type ObservableEntity = State;
+
+    fn observable_entity(&self) -> Option<gpui::Model<Self::ObservableEntity>> {
+        Some(self.state.clone())
     }
 }