chore: Make terminal_view own the TerminalSlashCommand (#31070)

This reduces 'touch crates/editor/src/editor.rs && cargo +nightly build'
from 8.9s to 8.5s. That same scenario used to take 8s less than a week
ago. :)
I'm measuring with nightly rustc, because it's compile times are better
than those of stable thanks to
https://github.com/rust-lang/rust/pull/138522

main (8.2s total):

![image](https://github.com/user-attachments/assets/767a2ac4-7bba-4147-bd16-9b09eed5b433)

[cargo-timing.html.zip](https://github.com/user-attachments/files/20364175/cargo-timing.html.zip)

#22be776 (7.5s total):

[cargo-timing-20250521T085303.892834Z.html.zip](https://github.com/user-attachments/files/20364391/cargo-timing-20250521T085303.892834Z.html.zip)

![image](https://github.com/user-attachments/assets/c4476df9-cb6e-4403-b0db-de00521f1fd0)


Release Notes:

- N/A

crates/agent/src/message_editor.rs

@@ -8,6 +8,7 @@ use crate::ui::{
     AnimatedLabel, MaxModeTooltip,
     preview::{AgentPreview, UsageCallout},
 };
+use assistant_context_editor::language_model_selector::ToggleModelSelector;
 use assistant_settings::{AssistantSettings, CompletionMode};
 use buffer_diff::BufferDiff;
 use client::UserStore;
@@ -30,7 +31,6 @@ use language_model::{
     ConfiguredModel, LanguageModelRequestMessage, MessageContent, RequestUsage,
     ZED_CLOUD_PROVIDER_ID,
 };
-use language_model_selector::ToggleModelSelector;
 use multi_buffer;
 use project::Project;
 use prompt_store::PromptStore;