Add refinements to the AI onboarding flow (#33738)

This includes making sure that both the agent panel and Zed's edit
prediction have a consistent narrative when it comes to onboarding users
into the AI features, considering the possible different plans and
conditions (such as being signed in/out, account age, etc.)

Release Notes:

- N/A

---------

Co-authored-by: Bennet Bo Fenner <53836821+bennetbo@users.noreply.github.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/agent_ui/src/ui.rs

@@ -1,6 +1,7 @@
 mod agent_notification;
 mod burn_mode_tooltip;
 mod context_pill;
+mod end_trial_upsell;
 mod onboarding_modal;
 pub mod preview;
 mod upsell;
@@ -8,4 +9,5 @@ mod upsell;
 pub use agent_notification::*;
 pub use burn_mode_tooltip::*;
 pub use context_pill::*;
+pub use end_trial_upsell::*;
 pub use onboarding_modal::*;