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 `///`).

desktop_photo_search/material/lib/src/model/photo_search_model.dart

@@ -45,5 +45,6 @@ class PhotoSearchModel extends ChangeNotifier {
     notifyListeners();
   }
 
-  Future<Uint8List> download({required Photo photo}) => _client.download(photo);
+  Future<Uint8List> download({required Photo photo}) =>
+      _client.download(photo);
 }

desktop_photo_search/material/lib/src/model/search.g.dart

@@ -33,7 +33,9 @@ class _$SearchSerializer implements StructuredSerializer<Search> {
       'results',
       serializers.serialize(
         object.results,
-        specifiedType: const FullType(BuiltList, const [const FullType(Photo)]),
+        specifiedType: const FullType(BuiltList, const [
+          const FullType(Photo),
+        ]),
       ),
     ];