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/outline_panel/src/outline_panel.rs

@@ -2785,7 +2785,7 @@ impl Panel for OutlinePanel {
         settings::update_settings_file::<OutlinePanelSettings>(
             self.fs.clone(),
             cx,
-            move |settings| {
+            move |settings, _| {
                 let dock = match position {
                     DockPosition::Left | DockPosition::Bottom => OutlinePanelDockPosition::Left,
                     DockPosition::Right => OutlinePanelDockPosition::Right,