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/lib/cascading_menu_page.dart

@@ -228,20 +228,22 @@ class _MyCascadingContextMenuState extends State<_MyCascadingContextMenu> {
           );
           widget.onToggleMessageVisibility();
         },
-        shortcut: const SingleActivator(LogicalKeyboardKey.keyS, control: true),
+        shortcut: const SingleActivator(
+          LogicalKeyboardKey.keyS,
+          control: true,
+        ),
       ),
       // Hides the message, but is only enabled if the message isn't
       // already hidden.
       MenuEntry(
         label: 'Reset',
-        onPressed:
-            widget.showingMessage
-                ? () {
-                  ContextMenuController.removeAny();
-                  widget.onChangeSelection('Reset');
-                  widget.onToggleMessageVisibility();
-                }
-                : null,
+        onPressed: widget.showingMessage
+            ? () {
+                ContextMenuController.removeAny();
+                widget.onChangeSelection('Reset');
+                widget.onToggleMessageVisibility();
+              }
+            : null,
         shortcut: const SingleActivator(LogicalKeyboardKey.escape),
       ),
       MenuEntry(