Remove assistant ContextSnapshot (#27822)

Motivation for this is to simplify the context types and make it cleaner
to add image context.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/assistant2/src/terminal_inline_assistant.rs

@@ -252,7 +252,8 @@ impl TerminalInlineAssistant {
 
         attach_context_to_message(
             &mut request_message,
-            assist.context_store.read(cx).snapshot(cx),
+            assist.context_store.read(cx).context().iter(),
+            cx,
         );
 
         request_message.content.push(prompt.into());