Add experimental LSP-based context retrieval system for edit prediction (#44036)

To do

* [x] Default to no context retrieval. Allow opting in to LSP-based
retrieval via a setting (for users in `zeta2` feature flag)
* [x] Feed this context to models when enabled
* [x] Make the zeta2 context view work well with LSP retrieval
* [x] Add a UI for the setting (for feature-flagged users)
* [x] Ensure Zeta CLI `context` command is usable

---

* [ ] Filter out LSP definitions that are too large / entire files (e.g.
modules)
* [ ] Introduce timeouts
* [ ] Test with other LSPs
* [ ] Figure out hangs

Release Notes:

- N/A

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>
Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/edit_prediction_button/src/edit_prediction_button.rs

@@ -1105,9 +1105,33 @@ impl EditPredictionButton {
                     .separator();
             }
 
-            let menu = self.build_language_settings_menu(menu, window, cx);
-            let menu = self.add_provider_switching_section(menu, provider, cx);
+            menu = self.build_language_settings_menu(menu, window, cx);
 
+            if cx.has_flag::<Zeta2FeatureFlag>() {
+                let settings = all_language_settings(None, cx);
+                let context_retrieval = settings.edit_predictions.use_context;
+                menu = menu.separator().header("Context Retrieval").item(
+                    ContextMenuEntry::new("Enable Context Retrieval")
+                        .toggleable(IconPosition::Start, context_retrieval)
+                        .action(workspace::ToggleEditPrediction.boxed_clone())
+                        .handler({
+                            let fs = self.fs.clone();
+                            move |_, cx| {
+                                update_settings_file(fs.clone(), cx, move |settings, _| {
+                                    settings
+                                        .project
+                                        .all_languages
+                                        .features
+                                        .get_or_insert_default()
+                                        .experimental_edit_prediction_context_retrieval =
+                                        Some(!context_retrieval)
+                                });
+                            }
+                        }),
+                );
+            }
+
+            menu = self.add_provider_switching_section(menu, provider, cx);
             menu
         })
     }