copilot: Decouple copilot sign in from edit prediction settings (#26689)

Closes #25883

This PR allows you to use copilot chat for assistant without setting
copilot as the edit prediction provider.


[copilot.webm](https://github.com/user-attachments/assets/fecfbde1-d72c-4c0c-b080-a07671fb846e)

Todos:
- [x] Remove redudant "copilot" key from settings
- [x] Do not disable copilot LSP when `edit_prediction_provider` is not
set to `copilot`
- [x] Start copilot LSP when:
  - [x]  `edit_prediction_provider` is set to `copilot`
  - [x] Copilot sign in clicked from assistant settings
- [x] Handle flicker for frame after starting LSP, but before signing in
caused due to signed out status
- [x] Fixed this by adding intermediate state for awaiting signing in in
sign out enum
- [x] Handle cancel button should sign out from `copilot` (existing bug)
- [x] Handle modal dismissal should sign out if not in signed in state
(existing bug)

Release Notes:

- You can now sign into Copilot from assistant settings without making
it your edit prediction provider. This is useful if you want to use
Copilot chat while keeping a different provider, like Zed, for
predictions.
- Removed the `copilot` key from `features` in settings. Use
`edit_prediction_provider` instead.

crates/language_models/src/provider/copilot_chat.rs

@@ -129,7 +129,7 @@ impl LanguageModelProvider for CopilotChatLanguageModelProvider {
             Status::Error(err) => anyhow!(format!("Received the following error while signing into Copilot: {err}")),
             Status::Starting { task: _ } => anyhow!("Copilot is still starting, please wait for Copilot to start then try again"),
             Status::Unauthorized => anyhow!("Unable to authorize with Copilot. Please make sure that you have an active Copilot and Copilot Chat subscription."),
-            Status::SignedOut => anyhow!("You have signed out of Copilot. Please sign in to Copilot and try again."),
+            Status::SignedOut {..} => anyhow!("You have signed out of Copilot. Please sign in to Copilot and try again."),
             Status::SigningIn { prompt: _ } => anyhow!("Still signing into Copilot..."),
         };
 
@@ -366,7 +366,6 @@ impl Render for ConfigurationView {
 
             match &self.copilot_status {
                 Some(status) => match status {
-                    Status::Disabled => v_flex().gap_6().p_4().child(Label::new(ERROR_LABEL)),
                     Status::Starting { task: _ } => {
                         const LABEL: &str = "Starting Copilot...";
                         v_flex()
@@ -376,7 +375,10 @@ impl Render for ConfigurationView {
                             .child(Label::new(LABEL))
                             .child(loading_icon)
                     }
-                    Status::SigningIn { prompt: _ } => {
+                    Status::SigningIn { prompt: _ }
+                    | Status::SignedOut {
+                        awaiting_signing_in: true,
+                    } => {
                         const LABEL: &str = "Signing in to Copilot...";
                         v_flex()
                             .gap_6()