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/assistant/src/context/context_tests.rs

@@ -17,7 +17,7 @@ use futures::{
     channel::mpsc,
     stream::{self, StreamExt},
 };
-use gpui::{AppContext, Model, SharedString, Task, TestAppContext, WeakView};
+use gpui::{prelude::*, AppContext, Model, SharedString, Task, TestAppContext, WeakView};
 use language::{Buffer, BufferSnapshot, LanguageRegistry, LspAdapterDelegate};
 use language_model::{LanguageModelCacheConfiguration, LanguageModelRegistry, Role};
 use parking_lot::Mutex;
@@ -35,7 +35,7 @@ use std::{
     sync::{atomic::AtomicBool, Arc},
 };
 use text::{network::Network, OffsetRangeExt as _, ReplicaId, ToOffset};
-use ui::{Context as _, IconName, WindowContext};
+use ui::{IconName, WindowContext};
 use unindent::Unindent;
 use util::{
     test::{generate_marked_text, marked_text_ranges},