gpui: Don't export named Context from prelude (#21869)

This PR updates the `gpui::prelude` to not export the `Context` trait
named.

This prevents some naming clashes in downstream consumers.

Release Notes:

- N/A

crates/language_model/src/registry.rs

@@ -3,9 +3,8 @@ use crate::{
     LanguageModelProviderState,
 };
 use collections::BTreeMap;
-use gpui::{AppContext, EventEmitter, Global, Model, ModelContext};
+use gpui::{prelude::*, AppContext, EventEmitter, Global, Model, ModelContext};
 use std::sync::Arc;
-use ui::Context;
 
 pub fn init(cx: &mut AppContext) {
     let registry = cx.new_model(|_cx| LanguageModelRegistry::default());