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/editor/src/editor.rs

@@ -10384,7 +10384,7 @@ impl Editor {
         };
         let fs = workspace.read(cx).app_state().fs.clone();
         let current_show = TabBarSettings::get_global(cx).show;
-        update_settings_file::<TabBarSettings>(fs, cx, move |setting| {
+        update_settings_file::<TabBarSettings>(fs, cx, move |setting, _| {
             setting.show = Some(!current_show);
         });
     }