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/lib/src/sign_in_http.dart

@@ -97,16 +97,15 @@ class _SignInHttpDemoState extends State<SignInHttpDemo> {
   void _showDialog(String message) {
     showDialog<void>(
       context: context,
-      builder:
-          (context) => AlertDialog(
-            title: Text(message),
-            actions: [
-              TextButton(
-                child: const Text('OK'),
-                onPressed: () => Navigator.of(context).pop(),
-              ),
-            ],
+      builder: (context) => AlertDialog(
+        title: Text(message),
+        actions: [
+          TextButton(
+            child: const Text('OK'),
+            onPressed: () => Navigator.of(context).pop(),
           ),
+        ],
+      ),
     );
   }
 }