assistant: Overhaul provider infrastructure (#14929)

<img width="624" alt="image"
src="https://github.com/user-attachments/assets/f492b0bd-14c3-49e2-b2ff-dc78e52b0815">

- [x] Correctly set custom model token count
- [x] How to count tokens for Gemini models?
- [x] Feature flag zed.dev provider
- [x] Figure out how to configure custom models
- [ ] Update docs

Release Notes:

- Added support for quickly switching between multiple language model
providers in the assistant panel

---------

Co-authored-by: Antonio <antonio@zed.dev>

crates/collab/src/tests/test_server.rs

@@ -157,6 +157,8 @@ impl TestServer {
     }
 
     pub async fn create_client(&mut self, cx: &mut TestAppContext, name: &str) -> TestClient {
+        let fs = FakeFs::new(cx.executor());
+
         cx.update(|cx| {
             if cx.has_global::<SettingsStore>() {
                 panic!("Same cx used to create two test clients")
@@ -265,7 +267,6 @@ impl TestServer {
         git_hosting_provider_registry
             .register_hosting_provider(Arc::new(git_hosting_providers::Github));
 
-        let fs = FakeFs::new(cx.executor());
         let user_store = cx.new_model(|cx| UserStore::new(client.clone(), cx));
         let workspace_store = cx.new_model(|cx| WorkspaceStore::new(client.clone(), cx));
         let language_registry = Arc::new(LanguageRegistry::test(cx.executor()));
@@ -297,7 +298,8 @@ impl TestServer {
             menu::init();
             dev_server_projects::init(client.clone(), cx);
             settings::KeymapFile::load_asset(os_keymap, cx).unwrap();
-            completion::FakeCompletionProvider::setup_test(cx);
+            language_model::LanguageModelRegistry::test(cx);
+            completion::init(cx);
             assistant::context_store::init(&client);
         });