fix(revert): Revert upgrade flagsmith-flag-engine to v10 (#6653)

[gagantrivedi]

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Changes

Reverts #6653

This PR reverts the flag-engine v10 upgrade, including:

• Reverting flagsmith-flag-engine from ^10.0.3 back to the fix/missing-export branch
• Reverting flagsmith SDK from ^5.1.1 back to ^3.10.0
• Removing pydantic-collections dependency
• Removing all vendored Pydantic models in util/engine_models/
• Restoring original imports across the codebase

How did you test this code?

• CI tests should pass

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Feb 10, 2026 0:38am
flagsmith-frontend-preview	Ignored	Preview	Feb 10, 2026 0:38am
flagsmith-frontend-staging	Ignored	Preview	Feb 10, 2026 0:38am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-6674	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-6674	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-6674	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-6674	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6674	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6674	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.20%. Comparing base (20215d5) to head (5af6124).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6674      +/-   ##
==========================================
- Coverage   98.22%   98.20%   -0.02%     
==========================================
  Files        1313     1298      -15     
  Lines       47481    47172     -309     
==========================================
- Hits        46636    46327     -309     
  Misses        845      845              

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