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/zed_actions/src/lib.rs

@@ -277,7 +277,10 @@ pub mod agent {
             /// Displays the previous message in the history.
             PreviousHistoryMessage,
             /// Displays the next message in the history.
-            NextHistoryMessage
+            NextHistoryMessage,
+            /// Toggles the language model selector dropdown.
+            #[action(deprecated_aliases = ["assistant::ToggleModelSelector", "assistant2::ToggleModelSelector"])]
+            ToggleModelSelector
         ]
     );
 }