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/custom_buttons_page_test.dart

@@ -37,7 +37,10 @@ void main() {
     expect(find.byType(AdaptiveTextSelectionToolbar), findsOneWidget);
     switch (defaultTargetPlatform) {
       case TargetPlatform.iOS:
-        expect(find.byType(CupertinoTextSelectionToolbarButton), findsNothing);
+        expect(
+          find.byType(CupertinoTextSelectionToolbarButton),
+          findsNothing,
+        );
         expect(find.byType(CupertinoButton), findsNWidgets(2));
       case TargetPlatform.macOS:
         expect(find.byType(CupertinoButton), findsNWidgets(2));
@@ -52,7 +55,10 @@ void main() {
       case TargetPlatform.linux:
       case TargetPlatform.windows:
         expect(find.byType(CupertinoButton), findsNWidgets(1));
-        expect(find.byType(DesktopTextSelectionToolbarButton), findsNothing);
+        expect(
+          find.byType(DesktopTextSelectionToolbarButton),
+          findsNothing,
+        );
     }
   });
 }