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 `///`).

background_isolate_channels/lib/simple_database.dart

@@ -188,7 +188,9 @@ class _SimpleDatabaseServer {
         // [BinaryMessenger] that the Platform Channels will communicate with on
         // the background isolate.
         // ----------------------------------------------------------------------
-        BackgroundIsolateBinaryMessenger.ensureInitialized(rootIsolateToken);
+        BackgroundIsolateBinaryMessenger.ensureInitialized(
+          rootIsolateToken,
+        );
         _sendPort.send(const _Command(_Codes.ack, arg0: null));
       case _Codes.add:
         _doAddEntry(command.arg0 as String);