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_ui/src/slash_command_picker.rs

@@ -155,8 +155,8 @@ impl PickerDelegate for SlashCommandDelegate {
             match command {
                 SlashCommandEntry::Info(info) => {
                     self.active_context_editor
-                        .update(cx, |context_editor, cx| {
-                            context_editor.insert_command(&info.name, window, cx)
+                        .update(cx, |text_thread_editor, cx| {
+                            text_thread_editor.insert_command(&info.name, window, cx)
                         })
                         .ok();
                 }