fix: Revert flag-engine v10 revert to re-apply engine upgrade

[gagantrivedi]

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Changes

Reverts #6674

This PR reverts the flag-engine v10 revert, effectively re-applying the original upgrade from #6653, including:

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

How did you test this code?

• CI tests should pass

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Project	Deployment	Actions	Updated (UTC)
flagsmith-frontend-preview	Ready	Preview, Comment	Feb 23, 2026 7:10pm
flagsmith-frontend-staging	Ready	Preview, Comment	Feb 23, 2026 7:10pm

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
docs	Ignored	Preview	Feb 23, 2026 7:10pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-6741	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-6741	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6741	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6741	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-6741	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api-test:pr-6741	Finished ✅	Skipped

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.26%. Comparing base (deee405) to head (70fe68c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6741      +/-   ##
==========================================
+ Coverage   98.25%   98.26%   +0.01%     
==========================================
  Files        1312     1327      +15     
  Lines       48568    48877     +309     
==========================================
+ Hits        47722    48031     +309     
  Misses        846      846              

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[Zaimwa9]

The branch is almost identical to the reverted one. Can I re-ask the reason why we reverted, was it out of safety during the hackaton or because we identified a regression?

[gagantrivedi]

The branch is almost identical to the reverted one. Can I re-ask the reason why we reverted, was it out of safety during the hackaton or because we identified a regression?

Yeah, it was because the e2e tests that you wrote for Edge, when running against staging, were failing. We later realised that the failure was caused by Core not having access to identity override data, and not because something was wrong with the evaluation engine/logic.