Fix duplicated Fix with Assistant code actions (#22911)

This PR fixes the duplicated `Fix with Assistant` code actions that were
being shown in the code actions menu.

This fix isn't 100% ideal, as there is an edge case in buffers that are
already open when the workspace loads, as we may not observe the feature
flags in time to register the code action providers by the time we
receive the event that an item was added to the workspace.

Closes https://github.com/zed-industries/zed/issues/22400.

Release Notes:

- Fixed duplicate "Fix with Assistant" entries showing in the code
action list.

crates/assistant2/src/inline_assistant.rs

@@ -19,6 +19,7 @@ use editor::{
     Anchor, AnchorRangeExt, CodeActionProvider, Editor, EditorEvent, ExcerptId, ExcerptRange,
     GutterDimensions, MultiBuffer, MultiBufferSnapshot, ToOffset as _, ToPoint,
 };
+use feature_flags::{Assistant2FeatureFlag, FeatureFlagViewExt as _};
 use fs::Fs;
 use util::ResultExt;
 
@@ -50,10 +51,17 @@ pub fn init(
 ) {
     cx.set_global(InlineAssistant::new(fs, prompt_builder, telemetry));
     cx.observe_new_views(|_workspace: &mut Workspace, cx| {
-        let workspace = cx.view().clone();
-        InlineAssistant::update_global(cx, |inline_assistant, cx| {
-            inline_assistant.register_workspace(&workspace, cx)
+        cx.observe_flag::<Assistant2FeatureFlag, _>({
+            |is_assistant2_enabled, _view, cx| {
+                if is_assistant2_enabled {
+                    let workspace = cx.view().clone();
+                    InlineAssistant::update_global(cx, |inline_assistant, cx| {
+                        inline_assistant.register_workspace(&workspace, cx)
+                    })
+                }
+            }
         })
+        .detach();
     })
     .detach();
 }