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/extensions_ui/src/extension_version_selector.rs

@@ -178,7 +178,7 @@ impl PickerDelegate for ExtensionVersionSelectorDelegate {
 
             update_settings_file::<ExtensionSettings>(self.fs.clone(), cx, {
                 let extension_id = extension_id.clone();
-                move |settings| {
+                move |settings, _| {
                     settings.auto_update_extensions.insert(extension_id, false);
                 }
             });

crates/extensions_ui/src/extensions_ui.rs

@@ -910,7 +910,7 @@ impl ExtensionsPage {
         if let Some(workspace) = self.workspace.upgrade() {
             let fs = workspace.read(cx).app_state().fs.clone();
             let selection = *selection;
-            settings::update_settings_file::<T>(fs, cx, move |settings| {
+            settings::update_settings_file::<T>(fs, cx, move |settings, _| {
                 let value = match selection {
                     Selection::Unselected => false,
                     Selection::Selected => true,