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 `///`).

context_menus/test/default_values_page_test.dart

@@ -55,7 +55,10 @@ void main() {
         );
       case TargetPlatform.android:
       case TargetPlatform.fuchsia:
-        expect(find.byType(TextSelectionToolbarTextButton), findsNWidgets(1));
+        expect(
+          find.byType(TextSelectionToolbarTextButton),
+          findsNWidgets(1),
+        );
       case TargetPlatform.linux:
       case TargetPlatform.windows:
         expect(