acp: Rename assistant::QuoteSelection and support it in agent2 threads (#36628)

Release Notes:

- N/A

crates/agent_ui/src/text_thread_editor.rs

@@ -1,4 +1,5 @@
 use crate::{
+    QuoteSelection,
     language_model_selector::{LanguageModelSelector, language_model_selector},
     ui::BurnModeTooltip,
 };
@@ -89,8 +90,6 @@ actions!(
         CycleMessageRole,
         /// Inserts the selected text into the active editor.
         InsertIntoEditor,
-        /// Quotes the current selection in the assistant conversation.
-        QuoteSelection,
         /// Splits the conversation at the current cursor position.
         Split,
     ]