Move UI code from assistant_context_editor -> agent_ui (#33289)

This breaks a transitive dependency of `agent` on UI crates. I've also
found and eliminated some dead code in assistant_context_editor.

Release Notes:

- N/A

crates/agent_ui/src/message_editor.rs

@@ -3,6 +3,7 @@ use std::rc::Rc;
 use std::sync::Arc;
 
 use crate::agent_model_selector::AgentModelSelector;
+use crate::language_model_selector::ToggleModelSelector;
 use crate::tool_compatibility::{IncompatibleToolsState, IncompatibleToolsTooltip};
 use crate::ui::{
     MaxModeTooltip,
@@ -13,7 +14,6 @@ use agent::{
     context_store::ContextStoreEvent,
 };
 use agent_settings::{AgentSettings, CompletionMode};
-use assistant_context_editor::language_model_selector::ToggleModelSelector;
 use buffer_diff::BufferDiff;
 use client::UserStore;
 use collections::{HashMap, HashSet};