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 `///`).

form_app/test/form_app_test.dart

@@ -22,7 +22,11 @@ void main() {
   });
 }
 
-Future<void> _signIn(WidgetTester tester, String email, String password) async {
+Future<void> _signIn(
+  WidgetTester tester,
+  String email,
+  String password,
+) async {
   await tester.pumpWidget(
     MaterialApp(home: SignInHttpDemo(httpClient: mockClient)),
   );