feat: DIA-1627: Support user context in LD

[niklub]

Extended get_user_repr() method to override LD context in LSE

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: label_studio/core/feature_flags/base.py

Function	Unhandled Issue
all_flags	AttributeError: 'NoneType' object has no attribute 'email' /p... Event Count: 1

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[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.77%. Comparing base (fc49438) to head (80b1d11).
Report is 18 commits behind head on develop.

Files with missing lines	Patch %	Lines
label_studio/core/feature_flags/utils.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #6685   +/-   ##
========================================
  Coverage    76.76%   76.77%           
========================================
  Files          169      170    +1     
  Lines        13865    13868    +3     
========================================
+ Hits         10644    10647    +3     
  Misses        3221     3221           

Flag	Coverage Δ
pytests	76.77% <92.85%> (+<0.01%)	⬆️

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