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/language/src/language_settings.rs

@@ -373,6 +373,8 @@ impl InlayHintSettings {
 pub struct EditPredictionSettings {
     /// The provider that supplies edit predictions.
     pub provider: settings::EditPredictionProvider,
+    /// Whether to use the experimental edit prediction context retrieval system.
+    pub use_context: bool,
     /// A list of globs representing files that edit predictions should be disabled for.
     /// This list adds to a pre-existing, sensible default set of globs.
     /// Any additional ones you add are combined with them.
@@ -622,6 +624,11 @@ impl settings::Settings for AllLanguageSettings {
             .features
             .as_ref()
             .and_then(|f| f.edit_prediction_provider);
+        let use_edit_prediction_context = all_languages
+            .features
+            .as_ref()
+            .and_then(|f| f.experimental_edit_prediction_context_retrieval)
+            .unwrap_or_default();
 
         let edit_predictions = all_languages.edit_predictions.clone().unwrap();
         let edit_predictions_mode = edit_predictions.mode.unwrap();
@@ -668,6 +675,7 @@ impl settings::Settings for AllLanguageSettings {
                 } else {
                     EditPredictionProvider::None
                 },
+                use_context: use_edit_prediction_context,
                 disabled_globs: disabled_globs
                     .iter()
                     .filter_map(|g| {