assistant2: Derive the Context icon at render time instead of storing (#22397)

This PR is a follow up to #22385 that makes it so we treat the icon as
derived state.

Release Notes:

- N/A

crates/assistant2/src/context.rs

@@ -1,7 +1,6 @@
 use gpui::SharedString;
 use language_model::{LanguageModelRequestMessage, MessageContent};
 use serde::{Deserialize, Serialize};
-use ui::prelude::*;
 use util::post_inc;
 
 #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, Serialize, Deserialize)]
@@ -20,10 +19,9 @@ pub struct Context {
     pub name: SharedString,
     pub kind: ContextKind,
     pub text: SharedString,
-    pub icon: IconName,
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum ContextKind {
     File,
     Directory,
@@ -31,17 +29,6 @@ pub enum ContextKind {
     Thread,
 }
 
-impl ContextKind {
-    pub fn icon(&self) -> IconName {
-        match self {
-            ContextKind::File => IconName::File,
-            ContextKind::Directory => IconName::Folder,
-            ContextKind::FetchedUrl => IconName::Globe,
-            ContextKind::Thread => IconName::MessageCircle,
-        }
-    }
-}
-
 pub fn attach_context_to_message(
     message: &mut LanguageModelRequestMessage,
     context: impl IntoIterator<Item = Context>,