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/inline_prompt_editor.rs

@@ -9,6 +9,7 @@ use crate::terminal_codegen::TerminalCodegen;
 use crate::thread_store::{TextThreadStore, ThreadStore};
 use crate::{CycleNextInlineAssist, CyclePreviousInlineAssist};
 use crate::{RemoveAllContext, ToggleContextPicker};
+use assistant_context_editor::language_model_selector::ToggleModelSelector;
 use client::ErrorExt;
 use collections::VecDeque;
 use db::kvp::Dismissable;
@@ -24,7 +25,6 @@ use gpui::{
     Focusable, FontWeight, Subscription, TextStyle, WeakEntity, Window, anchored, deferred, point,
 };
 use language_model::{LanguageModel, LanguageModelRegistry};
-use language_model_selector::ToggleModelSelector;
 use parking_lot::Mutex;
 use settings::Settings;
 use std::cmp;