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/terminal_view/src/terminal_panel.rs

@@ -760,14 +760,18 @@ impl Panel for TerminalPanel {
     }
 
     fn set_position(&mut self, position: DockPosition, cx: &mut ViewContext<Self>) {
-        settings::update_settings_file::<TerminalSettings>(self.fs.clone(), cx, move |settings| {
-            let dock = match position {
-                DockPosition::Left => TerminalDockPosition::Left,
-                DockPosition::Bottom => TerminalDockPosition::Bottom,
-                DockPosition::Right => TerminalDockPosition::Right,
-            };
-            settings.dock = Some(dock);
-        });
+        settings::update_settings_file::<TerminalSettings>(
+            self.fs.clone(),
+            cx,
+            move |settings, _| {
+                let dock = match position {
+                    DockPosition::Left => TerminalDockPosition::Left,
+                    DockPosition::Bottom => TerminalDockPosition::Bottom,
+                    DockPosition::Right => TerminalDockPosition::Right,
+                };
+                settings.dock = Some(dock);
+            },
+        );
     }
 
     fn size(&self, cx: &WindowContext) -> Pixels {