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/agent_panel.rs

@@ -17,6 +17,7 @@ use assistant_settings::{AssistantDockPosition, AssistantSettings};
 use assistant_slash_command::SlashCommandWorkingSet;
 use assistant_tool::ToolWorkingSet;
 
+use assistant_context_editor::language_model_selector::ToggleModelSelector;
 use client::{UserStore, zed_urls};
 use editor::{Anchor, AnchorRangeExt as _, Editor, EditorEvent, MultiBuffer};
 use fs::Fs;
@@ -30,7 +31,6 @@ use language::LanguageRegistry;
 use language_model::{
     LanguageModelProviderTosView, LanguageModelRegistry, RequestUsage, ZED_CLOUD_PROVIDER_ID,
 };
-use language_model_selector::ToggleModelSelector;
 use project::{Project, ProjectPath, Worktree};
 use prompt_store::{PromptBuilder, PromptStore, UserPromptId};
 use proto::Plan;