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/collab/Cargo.toml

@@ -78,7 +78,7 @@ zed_llm_client.workspace = true
 
 [dev-dependencies]
 agent_settings.workspace = true
-assistant_context_editor.workspace = true
+assistant_context.workspace = true
 assistant_slash_command.workspace = true
 async-trait.workspace = true
 audio.workspace = true

crates/collab/src/tests/integration_tests.rs

@@ -6,7 +6,7 @@ use crate::{
     },
 };
 use anyhow::{Result, anyhow};
-use assistant_context_editor::ContextStore;
+use assistant_context::ContextStore;
 use assistant_slash_command::SlashCommandWorkingSet;
 use buffer_diff::{DiffHunkSecondaryStatus, DiffHunkStatus, assert_hunks};
 use call::{ActiveCall, ParticipantLocation, Room, room};

crates/collab/src/tests/test_server.rs

@@ -313,7 +313,7 @@ impl TestServer {
                 settings::KeymapFile::load_asset_allow_partial_failure(os_keymap, cx).unwrap(),
             );
             language_model::LanguageModelRegistry::test(cx);
-            assistant_context_editor::init(client.clone(), cx);
+            assistant_context::init(client.clone(), cx);
             agent_settings::init(cx);
         });