refactor(ui): update ReactionIconResolver to return StreamEmojiContent instead of Widget

[xsahil03x]

Description of the pull request

• Breaking Change: Modified ReactionIconResolver.resolve signature to return StreamEmojiContent and removed the BuildContext parameter.
• Updated DefaultReactionIconResolver to return StreamUnicodeEmoji by default.
• Updated StreamMessageReactionPicker, StreamMessageReactions, ReactionDetailSheet, and StreamMessageReactions to use the new data-driven resolution logic.
• Refactored DesktopReactionsBuilder to use StreamEmoji with the new resolver output.
• Updated migration documentation and tests to reflect the shift from widget-based resolution to content-model resolution (supporting StreamImageEmoji for custom rendering like Twemoji).
• Adjusted StreamGalleryPicker to use a transparent background for the overlay color.
• Temporarily updated stream_core_flutter dependency to a local path.

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.32%. Comparing base (cd3cd88) to head (7a6302e).
⚠️ Report is 1 commits behind head on feat/design-refresh.

Files with missing lines	Patch %	Lines
...ge_widget/components/stream_message_reactions.dart	0.00%	1 Missing ⚠️
...chat_flutter/lib/src/reactions/user_reactions.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##           feat/design-refresh    #2591   +/-   ##
====================================================
  Coverage                65.32%   65.32%           
====================================================
  Files                      429      429           
  Lines                    26137    26133    -4     
====================================================
- Hits                     17073    17071    -2     
+ Misses                    9064     9062    -2     

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