refactor(ui): migrate from golden_toolkit -> alchemist

[xsahil03x]

This pull request includes several changes to improve the testing framework and dependencies in the stream_chat_flutter package. The most important changes include replacing the golden_toolkit package with the alchemist package for golden tests and making minor adjustments to widget properties.

Dependency Updates:

• Replaced golden_toolkit with alchemist in dev_dependencies across multiple files, including melos.yaml and pubspec.yaml. [1] [2]

Testing Framework Changes:

• Updated import statements to use alchemist instead of golden_toolkit in various test files. [1] [2] [3] [4] [5] [6] [7] [8]
• Replaced testGoldens with goldenTest and adjusted test configurations accordingly in multiple test files. [1] [2] [3] [4] [5] [6] [7] [8] [9]

Widget Adjustments:

• Added mainAxisSize: MainAxisSize.min to Row widget in dm_checkbox.dart to ensure minimal horizontal space usage.
• Added crossAxisAlignment: CrossAxisAlignment.start to Column widget in stream_message_input.dart to align children to the start.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.14%. Comparing base (e2f0177) to head (2ecf41b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2070      +/-   ##
==========================================
+ Coverage   61.03%   61.14%   +0.11%     
==========================================
  Files         336      336              
  Lines       20152    20152              
==========================================
+ Hits        12299    12322      +23     
+ Misses       7853     7830      -23     

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