assistant2: Suggest recent files and threads as context (#22959)

The context picker will now display up to 6 recent files/threads to add
as a context:

<img
src="https://github.com/user-attachments/assets/80c87bf9-70ad-4e81-ba24-7a624378b991"
width=400>



Note: We decided to use a `ContextMenu` instead of `Picker` for the
initial one since the latter didn't quite fit the design for the
"Recent" section.

Release Notes:

- N/A

---------

Co-authored-by: Danilo <danilo@zed.dev>
Co-authored-by: Piotr <piotr@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>

crates/assistant2/src/context.rs

@@ -43,6 +43,24 @@ pub enum ContextKind {
 }
 
 impl ContextKind {
+    pub fn all() -> &'static [ContextKind] {
+        &[
+            ContextKind::File,
+            ContextKind::Directory,
+            ContextKind::FetchedUrl,
+            ContextKind::Thread,
+        ]
+    }
+
+    pub fn label(&self) -> &'static str {
+        match self {
+            ContextKind::File => "File",
+            ContextKind::Directory => "Folder",
+            ContextKind::FetchedUrl => "Fetch",
+            ContextKind::Thread => "Thread",
+        }
+    }
+
     pub fn icon(&self) -> IconName {
         match self {
             ContextKind::File => IconName::File,