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/agent/src/native_agent_server.rs

@@ -81,7 +81,7 @@ impl AgentServer for NativeAgentServer {
 mod tests {
     use super::*;
 
-    use assistant_context::ContextStore;
+    use assistant_text_thread::TextThreadStore;
     use gpui::AppContext;
 
     agent_servers::e2e_tests::common_e2e_tests!(
@@ -116,8 +116,9 @@ mod tests {
             });
 
             let history = cx.update(|cx| {
-                let context_store = cx.new(move |cx| ContextStore::fake(project.clone(), cx));
-                cx.new(move |cx| HistoryStore::new(context_store, cx))
+                let text_thread_store =
+                    cx.new(move |cx| TextThreadStore::fake(project.clone(), cx));
+                cx.new(move |cx| HistoryStore::new(text_thread_store, cx))
             });
 
             NativeAgentServer::new(fs.clone(), history)