chore: Move SegmentSerializer and MetadataSerializer back to core API

[emyller]

Contributes to #5769.

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

• Move SegmentSerializer, RuleSerializer, and ConditionSerializer back to the core API code. They were moved in the past, and have since evolved.
• Delete custom logic to handle nested objects in favor of WritableNestedModelSerializer.

How did you test this code?

Replaced multiple organs, patient walked out smiling.

• Double check test coverage.

[vercel]

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Aug 5, 2025 7:22pm
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		Aug 5, 2025 7:22pm
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		Aug 5, 2025 7:22pm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.83%. Comparing base (63f943a) to head (a36ed53).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5775   +/-   ##
=======================================
  Coverage   97.82%   97.83%           
=======================================
  Files        1258     1259    +1     
  Lines       44846    44904   +58     
=======================================
+ Hits        43872    43932   +60     
+ Misses        974      972    -2     

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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-5775	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-5775	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-5775	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-5775	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-5775	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5775	Finished ✅	Results ✅

[matthewelwell]

Looks pretty good on the whole - definitely cleaner than it was, so nice work! I've added a couple of questions, but nothing that I'd be concerned will present any blockers.