feat(ui): factory slots for message item leading, header, footer

[xsahil03x]

Summary

• Adds per-slot factory overrides for the message item's leading, header, and footer sub-components. Previously, the only override was messageItem, which forced integrators to replace the entire item even when they only wanted to swap the avatar, the pinned/reminder annotations, or the timestamp row.
• Closes a parity gap with the other SDKs that already expose this:
  • iOS SwiftUI — makeUserAvatarView (ViewFactory.swift:120-123)
  • Android Compose — MessageAuthor / MessageTop / MessageBottom on ChatComponentFactory
  • React Native — MessageAuthor / MessageHeader / MessageFooter on the components context
• Motivated by docs-content PR #1241 review — Martin's question about a leading/trailing customization path.

What's in this PR

Commit 1 — refactor(ui): extract StreamMessageLeading from message item

Extracts the message item's inline avatar block (stream_message_item.dart:433-443) into a new StreamMessageLeading widget under components/, mirroring the existing StreamMessageHeader and StreamMessageFooter. DefaultStreamMessageItem now composes leading, header, and footer symmetrically.

Commit 2 — feat(ui): factory slots for message item leading, header, footer

Refactors each of the three sub-components into the StreamMessageItem shape (public widget → Default… impl → …Props) and registers three new keys in streamChatComponentBuilders:

StreamComponentBuilder<StreamMessageLeadingProps>? messageLeading,
StreamComponentBuilder<StreamMessageHeaderProps>? messageHeader,
StreamComponentBuilder<StreamMessageFooterProps>? messageFooter,

The widgets are also exported from the package barrel so integrators can reference the props and default classes from their custom builders.

Usage

StreamChat(
  client: client,
  componentBuilders: StreamComponentBuilders(
    extensions: streamChatComponentBuilders(
      // Swap just the avatar.
      messageLeading: (context, props) =>
          MyPresenceAvatar(user: props.message.user),
      // Add a custom annotation while keeping the defaults.
      messageHeader: (context, props) =>
          DefaultStreamMessageHeader(props: props),
      // Replace the timestamp row entirely.
      messageFooter: (context, props) =>
          MyCustomFooter(message: props.message),
    ),
  ),
  child: MyApp(),
)

Each slot is independently overridable — none of them require touching the whole messageItem.

Test plan

• dart analyze --fatal-infos packages/stream_chat_flutter — clean
• dart format — clean (page width 120)
• melos run test:flutter — golden tests for StreamMessageItem still match (no visual change expected; local run shows 115 pre-existing macOS golden mismatches unrelated to this change, none referencing the touched files)
• Spot-check sample app: 1:1 channel (no leading avatar, no header), group channel own message (no leading avatar, footer with status/time), group channel other-user message (leading avatar at bottom of grouped run, header/footer as before)
• Verify each slot in isolation by registering a custom builder in the sample app

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 63.26531% with 18 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (v10.0.0@d9b3105). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ssage_widget/components/stream_message_header.dart	53.33%	7 Missing ⚠️
...ssage_widget/components/stream_message_footer.dart	63.63%	4 Missing ⚠️
...sage_widget/components/stream_message_leading.dart	76.47%	4 Missing ⚠️
...src/components/stream_chat_component_builders.dart	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             v10.0.0    #2710   +/-   ##
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  Coverage           ?   67.59%           
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  Files              ?      408           
  Lines              ?    24506           
  Branches           ?        0           
==========================================
  Hits               ?    16564           
  Misses             ?     7942           
  Partials           ?        0           

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