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/theme_selector/src/theme_selector.rs

@@ -196,7 +196,7 @@ impl PickerDelegate for ThemeSelectorDelegate {
 
         let appearance = Appearance::from(cx.appearance());
 
-        update_settings_file::<ThemeSettings>(self.fs.clone(), cx, move |settings| {
+        update_settings_file::<ThemeSettings>(self.fs.clone(), cx, move |settings, _| {
             if let Some(selection) = settings.theme.as_mut() {
                 let theme_to_update = match selection {
                     ThemeSelection::Static(theme) => theme,