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/client/src/user.rs

@@ -764,6 +764,16 @@ impl UserStore {
     }
 
     pub fn current_plan(&self) -> Option<proto::Plan> {
+        #[cfg(debug_assertions)]
+        if let Ok(plan) = std::env::var("ZED_SIMULATE_ZED_PRO_PLAN").as_ref() {
+            return match plan.as_str() {
+                "free" => Some(proto::Plan::Free),
+                "trial" => Some(proto::Plan::ZedProTrial),
+                "pro" => Some(proto::Plan::ZedPro),
+                _ => None,
+            };
+        }
+
         self.current_plan
     }