fix(ui): auto-scroll on optimistic local messages

Subscribe to `channel.state.messagesStream` (or `threadsStream[parentId]`
in thread mode) instead of `channel.on(EventType.messageNew)` so the list
follows to the new bottom message the moment it lands in state. The event
path only fires on server-confirmed messages, which meant the user's own
send wasn't auto-scrolled until the server round-trip completed.

The data-source-driven pattern matches what the Android, iOS, and React
Native SDKs do. New-message detection uses a `lengthGrew && lastChanged`
check between emissions; the bottom-most snapshot is seeded from current
state on subscribe so we don't auto-scroll on the BehaviorSubject replay.
The synchronous `controller.scrollTo(index: 0)` call still clears SPL's
anchor key before `didUpdateWidget` (no race).

Adds an `auto_scroll_test.dart` covering: other-user-at-bottom,
other-user-scrolled-up, own-message-scrolled-up, optimistic local send,
and rapid burst.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: xsahil03x

packages/stream_chat_flutter/CHANGELOG.md

@@ -1,3 +1,10 @@
+## Upcoming Changes
+
+🐞 Fixed
+
+- Fixed `StreamMessageListView` not auto-scrolling to the bottom on the user's own outgoing message
+  until the server confirmed it.
+
 ## 9.24.0
 
 ✅ Added