feat: ✨ Add headerSlivers support to ChatList for advanced pinned headers

[KiddoV]

Summary

This PR adds a new optional headerSlivers parameter to ChatList, allowing consumers to provide multiple slivers (e.g. SliverAppBar, SliverPersistentHeader) before the chat list. This make the ChatList more customizable.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

[Copilot]

Pull request overview

Adds a new headerSlivers parameter to ChatList so consumers can insert multiple sliver-based headers (e.g., SliverAppBar, SliverPersistentHeader) above the chat list content for more advanced/pinned header layouts.

Changes:

• Introduced optional headerSlivers on ChatList.
• Rendered headerSlivers in CustomScrollView.slivers before the existing header.
• Documented the feature in CHANGELOG.md.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/src/widgets/chat_list/chat_list.dart	Adds headerSlivers API and spreads provided slivers into the CustomScrollView before the chat list.
CHANGELOG.md	Records the new headerSlivers feature in the Unreleased section.

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[Copilot]

headerSlivers is typed as List<Widget>, but CustomScrollView.slivers only accepts widgets that build a RenderSliver. If a consumer passes a regular box widget (e.g. Container) it will throw at runtime. Consider tweaking the dartdoc to explicitly state each entry must be a sliver widget (or should be wrapped in SliverToBoxAdapter) to avoid misuse.

Suggested change

	/// List of sliver headers to be displayed at the top of the chat list.
	/// List of sliver headers to be displayed at the top of the chat list.
	///
	/// Each widget in this list must be a sliver widget that builds a
	/// [RenderSliver] (e.g. [SliverAppBar], [SliverPersistentHeader],
	/// [SliverList], etc.). If you want to use a regular box widget
	/// (such as [Container] or [SizedBox]) as a header, wrap it in a
	/// [SliverToBoxAdapter] before adding it to this list. These widgets
	/// are inserted into the [CustomScrollView.slivers] used by [ChatList].