agent: Support inserting selections as context via @selection (#29045)

WIP

Release Notes:

- N/A

crates/agent/src/message_editor.rs

@@ -298,7 +298,7 @@ impl MessageEditor {
                         .filter(|ctx| {
                             matches!(
                                 ctx,
-                                AssistantContext::Excerpt(_) | AssistantContext::Image(_)
+                                AssistantContext::Selection(_) | AssistantContext::Image(_)
                             )
                         })
                         .map(|ctx| ctx.id())