agent2: Port Zed AI features (#36172)

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/agent_ui/src/text_thread_editor.rs

@@ -1,6 +1,6 @@
 use crate::{
-    burn_mode_tooltip::BurnModeTooltip,
     language_model_selector::{LanguageModelSelector, language_model_selector},
+    ui::BurnModeTooltip,
 };
 use agent_settings::{AgentSettings, CompletionMode};
 use anyhow::Result;