fix: Add missing Swagger schema for GET feature by ID

[rolodato]

Thanks for submitting a PR! Please check the boxes below:

• 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

Currently, there is no response schema for the "GET feature by ID" endpoint https://api.flagsmith.com/api/v1/docs/#/api/api_v1_projects_features_read

This PR adds the correct response schema:

How did you test this code?

DEBUG=True python manage.py runserver

then browse to http://127.0.0.1:8000/api/v1/docs/#/api/api_v1_projects_features_read

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Jul 8, 2025 0:26am
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		Jul 8, 2025 0:26am
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		Jul 8, 2025 0:26am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-5713	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-5713	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-5713	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-5713	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5713	Finished ✅	Results ✅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.77%. Comparing base (1e580e5) to head (77eb346).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5713   +/-   ##
=======================================
  Coverage   97.77%   97.77%           
=======================================
  Files        1253     1255    +2     
  Lines       44275    44351   +76     
=======================================
+ Hits        43289    43365   +76     
  Misses        986      986           

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