Add option for code context menu items to have dynamic width (#37404)

Follow up to https://github.com/zed-industries/zed/pull/30598

This PR introduces the `display_options` field in the
`CompletionResponse`, allowing a code context menu width to be
dynamically dictated based on its larger item. This will allow us to
have the @-mentions and slash commands completion menus in the agent
panel not be bigger than it needs to be. It may also be relevant/useful
in the future for other use cases.

For now, we set all instances of code context menus to use a fixed
width, as defined in the PR linked above, which means this PR shouldn't
cause any visual change.

Release Notes:

- N/A

Co-authored-by: Michael Sloan <mgsloan+github@gmail.com>

crates/keymap_editor/src/keymap_editor.rs

@@ -22,7 +22,7 @@ use gpui::{
 };
 use language::{Language, LanguageConfig, ToOffset as _};
 use notifications::status_toast::{StatusToast, ToastIcon};
-use project::Project;
+use project::{CompletionDisplayOptions, Project};
 use settings::{BaseKeymap, KeybindSource, KeymapFile, Settings as _, SettingsAssets};
 use ui::{
     ActiveTheme as _, App, Banner, BorrowAppContext, ContextMenu, IconButtonShape, Indicator,
@@ -2911,6 +2911,7 @@ impl CompletionProvider for KeyContextCompletionProvider {
                     confirm: None,
                 })
                 .collect(),
+            display_options: CompletionDisplayOptions::default(),
             is_incomplete: false,
         }]))
     }