completions: Add subtle/eager behavior to Supermaven and Copilot (#35548)

This pull request introduces changes to improve the behavior and
consistency of multiple completion providers
(`CopilotCompletionProvider`, `SupermavenCompletionProvider`) and their
integration with UI elements like menus and inline completion buttons.
It now allows to see the prediction with the completion menu open whilst
pressing `opt` and also enables the subtle/eager setting that was
introduced with zeta.

Edit: I managed to get the preview working with correct icons!
<img width="909" height="232" alt="image"
src="https://github.com/user-attachments/assets/65800e67-4bc4-40f8-be78-806fcfe74ad9"
/>
<img width="1460" height="318" alt="CleanShot 2025-08-04 at 01 36 31@2x"
src="https://github.com/user-attachments/assets/15651405-720f-465f-a13c-c7470817810a"
/>

Correct icons are also displayed:
<img width="244" height="96" alt="image"
src="https://github.com/user-attachments/assets/0b8a687f-73e3-452d-aefb-784c52831b73"
/>


Edit2: I added some comments, would be very happy to receive feedback
(still learning rust)

Release Notes:

- Added Subtle and Eager edit prediction modes to Copilot and Supermaven

crates/edit_prediction_button/src/edit_prediction_button.rs

@@ -491,7 +491,12 @@ impl EditPredictionButton {
         let subtle_mode = matches!(current_mode, EditPredictionsMode::Subtle);
         let eager_mode = matches!(current_mode, EditPredictionsMode::Eager);
 
-        if matches!(provider, EditPredictionProvider::Zed) {
+        if matches!(
+            provider,
+            EditPredictionProvider::Zed
+                | EditPredictionProvider::Copilot
+                | EditPredictionProvider::Supermaven
+        ) {
             menu = menu
                 .separator()
                 .header("Display Modes")