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/zed/src/zed.rs

@@ -3436,6 +3436,7 @@ mod tests {
             project_panel::init((), cx);
             outline_panel::init((), cx);
             terminal_view::init(cx);
+            language_model::init(app_state.client.clone(), cx);
             assistant::init(app_state.fs.clone(), app_state.client.clone(), cx);
             repl::init(app_state.fs.clone(), cx);
             tasks_ui::init(cx);