Dart 3.9 / Flutter 3.35 [first LLM release] (#2714)

I got carried away with Gemini and basically rewrote CI and the release
process for the new LLM reality. This work was largely completed by
Gemini.

- Bump all SDK versions to the current beta (3.9.0-0)
- Run `flutter channel beta`
- Wrote `ci_script.dart` to replace the bash scripts
- Converted repository to pub workspace #2499 
- Added llm.md and release.md
- Added redirect for deprecated Samples Index

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

provider_shopper/lib/models/cart.dart

@@ -24,7 +24,8 @@ class CartModel extends ChangeNotifier {
   }
 
   /// List of items in the cart.
-  List<Item> get items => _itemIds.map((id) => _catalog.getById(id)).toList();
+  List<Item> get items =>
+      _itemIds.map((id) => _catalog.getById(id)).toList();
 
   /// The current total price of all items.
   int get totalPrice =>