chore: Allow cypress tests to set feature flags in tests explicitly

[bmartel]

This pull request updates the feature flag resolution logic in label_studio/core/feature_flags/base.py to clarify and enforce the priority order when determining the value of a feature flag. The changes ensure that feature flags are checked in a consistent sequence, making it easier to understand and debug feature flag behavior, especially for testing and deployment environments.

Priority order and logic improvements:

• Added a clear comment documenting the priority order for feature flag resolution: stale flags, environment variables, Cypress Redis overrides, LaunchDarkly, and default values.
• Moved and refactored the environment variable check to ensure it is evaluated immediately after stale flags, enforcing the documented priority.
• Introduced logic to support Cypress Redis feature flag overrides in CYPRESS_ENVIRONMENT, allowing E2E tests to dynamically override flags using session-scoped values.
• Improved error handling for Cypress overrides to gracefully handle missing modules or runtime errors, logging warnings where appropriate.

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[codecov]

Codecov Report

❌ Patch coverage is 22.22222% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.28%. Comparing base (93b7f49) to head (41e52c0).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
label_studio/core/feature_flags/base.py	22.22%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #8967       +/-   ##
============================================
+ Coverage    66.59%   81.28%   +14.68%     
============================================
  Files          828      267      -561     
  Lines        64602    24061    -40541     
  Branches     10986        0    -10986     
============================================
- Hits         43023    19558    -23465     
+ Misses       21575     4503    -17072     
+ Partials         4        0        -4     

Flag	Coverage Δ
lsf-e2e	?
lsf-integration	?
lsf-unit	?
pytests	81.28% <22.22%> (-0.03%)	⬇️

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