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 `///`).

infinite_list/test/smoke_test.dart

@@ -25,7 +25,11 @@ void main() {
     expect(find.text('Color #1'), findsOneWidget);
 
     // Flinging up quickly (i.e. scrolling down).
-    await tester.fling(find.byType(ListView), const Offset(0, -2000), 5000);
+    await tester.fling(
+      find.byType(ListView),
+      const Offset(0, -2000),
+      5000,
+    );
 
     // As we scroll down, we should see more items loading.
     expect(find.text('...'), findsWidgets);