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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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored		Feb 20, 2026 7:49am
flagsmith-frontend-preview	Ignored		Feb 20, 2026 7:49am
flagsmith-frontend-staging	Ignored		Feb 20, 2026 7:49am

[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-api-test:pr-6741	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith:pr-6741	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6741	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.24%. Comparing base (7c0bf27) to head (fdd9cad).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6741      +/-   ##
==========================================
+ Coverage   98.23%   98.24%   +0.01%     
==========================================
  Files        1312     1327      +15     
  Lines       48490    48799     +309     
==========================================
+ Hits        47633    47942     +309     
  Misses        857      857              

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