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/buffer_codegen.rs

@@ -414,7 +414,11 @@ impl CodegenAlternative {
         };
 
         if let Some(context_store) = &self.context_store {
-            attach_context_to_message(&mut request_message, context_store.read(cx).snapshot(cx));
+            attach_context_to_message(
+                &mut request_message,
+                context_store.read(cx).context().iter(),
+                cx,
+            );
         }
 
         request_message.content.push(prompt.into());