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/copilot/src/copilot_completion_provider.rs

@@ -58,11 +58,19 @@ impl EditPredictionProvider for CopilotCompletionProvider {
     }
 
     fn show_completions_in_menu() -> bool {
+        true
+    }
+
+    fn show_tab_accept_marker() -> bool {
+        true
+    }
+
+    fn supports_jump_to_edit() -> bool {
         false
     }
 
     fn is_refreshing(&self) -> bool {
-        self.pending_refresh.is_some()
+        self.pending_refresh.is_some() && self.completions.is_empty()
     }
 
     fn is_enabled(
@@ -343,8 +351,8 @@ mod tests {
         executor.advance_clock(COPILOT_DEBOUNCE_TIMEOUT);
         cx.update_editor(|editor, window, cx| {
             assert!(editor.context_menu_visible());
-            assert!(!editor.has_active_edit_prediction());
-            // Since we have both, the copilot suggestion is not shown inline
+            assert!(editor.has_active_edit_prediction());
+            // Since we have both, the copilot suggestion is existing but does not show up as ghost text
             assert_eq!(editor.text(cx), "one.\ntwo\nthree\n");
             assert_eq!(editor.display_text(cx), "one.\ntwo\nthree\n");
 
@@ -934,8 +942,9 @@ mod tests {
         executor.advance_clock(COPILOT_DEBOUNCE_TIMEOUT);
         cx.update_editor(|editor, _, cx| {
             assert!(editor.context_menu_visible());
-            assert!(!editor.has_active_edit_prediction(),);
+            assert!(editor.has_active_edit_prediction());
             assert_eq!(editor.text(cx), "one\ntwo.\nthree\n");
+            assert_eq!(editor.display_text(cx), "one\ntwo.\nthree\n");
         });
     }
 
@@ -1077,8 +1086,6 @@ mod tests {
             vec![complete_from_marker.clone(), replace_range_marker.clone()],
         );
 
-        let complete_from_position =
-            cx.to_lsp(marked_ranges.remove(&complete_from_marker).unwrap()[0].start);
         let replace_range =
             cx.to_lsp_range(marked_ranges.remove(&replace_range_marker).unwrap()[0].clone());
 
@@ -1087,10 +1094,6 @@ mod tests {
                 let completions = completions.clone();
                 async move {
                     assert_eq!(params.text_document_position.text_document.uri, url.clone());
-                    assert_eq!(
-                        params.text_document_position.position,
-                        complete_from_position
-                    );
                     Ok(Some(lsp::CompletionResponse::Array(
                         completions
                             .iter()