Rename assistant_context crate to assistant_text_thread (#41024)

Previously we had `Context` and `ContextStore` in both `agent_ui` (used
to store context for the inline assistant) and `assistant_context` (used
for text threads) which is confusing.
This PR makes it so that the `assistant_context` concepts are now called
`TextThread*`, the crate was renamed to `assistant_text_thread`

Release Notes:

- N/A

crates/assistant_text_thread/src/assistant_text_thread.rs

@@ -0,0 +1,15 @@
+#[cfg(test)]
+mod assistant_text_thread_tests;
+mod text_thread;
+mod text_thread_store;
+
+pub use crate::text_thread::*;
+pub use crate::text_thread_store::*;
+
+use client::Client;
+use gpui::App;
+use std::sync::Arc;
+
+pub fn init(client: Arc<Client>, _: &mut App) {
+    text_thread_store::init(&client.into());
+}