Wire up @mentioned files in assistant2 (#23389)

`@mention`ed files in assistant2 now get replaced by the full path of
the file in what gets sent to the model, while rendering visually as
just the filename (in a crease, so they can only be selected/deleted as
a whole unit, not character by character).


https://github.com/user-attachments/assets/a5867a93-d656-4a17-aced-58424c6e8cf6

Release Notes:

- N/A

---------

Co-authored-by: João Marcos <joao@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -8,7 +8,7 @@ pub use crate::slash_command_working_set::*;
 use anyhow::Result;
 use futures::stream::{self, BoxStream};
 use futures::StreamExt;
-use gpui::{AnyElement, AppContext, ElementId, SharedString, Task, WeakView, WindowContext};
+use gpui::{AppContext, SharedString, Task, WeakView, WindowContext};
 use language::{BufferSnapshot, CodeLabel, LspAdapterDelegate, OffsetRangeExt};
 pub use language_model::Role;
 use serde::{Deserialize, Serialize};
@@ -103,12 +103,6 @@ pub trait SlashCommand: 'static + Send + Sync {
     ) -> Task<SlashCommandResult>;
 }
 
-pub type RenderFoldPlaceholder = Arc<
-    dyn Send
-        + Sync
-        + Fn(ElementId, Arc<dyn Fn(&mut WindowContext)>, &mut WindowContext) -> AnyElement,
->;
-
 #[derive(Debug, PartialEq)]
 pub enum SlashCommandContent {
     Text {