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/agent_ui/src/context_picker/completion_provider.rs

@@ -13,7 +13,10 @@ use http_client::HttpClientWithUrl;
 use itertools::Itertools;
 use language::{Buffer, CodeLabel, HighlightId};
 use lsp::CompletionContext;
-use project::{Completion, CompletionIntent, CompletionResponse, ProjectPath, Symbol, WorktreeId};
+use project::{
+    Completion, CompletionDisplayOptions, CompletionIntent, CompletionResponse, ProjectPath,
+    Symbol, WorktreeId,
+};
 use prompt_store::PromptStore;
 use rope::Point;
 use text::{Anchor, OffsetRangeExt, ToPoint};
@@ -897,6 +900,7 @@ impl CompletionProvider for ContextPickerCompletionProvider {
 
             Ok(vec![CompletionResponse {
                 completions,
+                display_options: CompletionDisplayOptions::default(),
                 // Since this does its own filtering (see `filter_completions()` returns false),
                 // there is no benefit to computing whether this set of completions is incomplete.
                 is_incomplete: true,