fix: CORS errors when sending tracked headers other than Flagsmith-SDK-User-Agent

[khvn26]

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

• I have added information to docs/ if required so people know about the feature!
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• 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

This PR is a continuation of #6010. It includes other tracked headers to CORS exclusion list, and makes sure that each header is included only once.

How did you test this code?

Tested locally, and will test on staging.

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3 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
docs	Ignored			Sep 3, 2025 3:24pm
flagsmith-frontend-preview	Ignored			Sep 3, 2025 3:24pm
flagsmith-frontend-staging	Ignored			Sep 3, 2025 3:24pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-6017	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-6017	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-6017	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6017	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6017	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-6017	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.86%. Comparing base (332a2df) to head (13e72c9).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6017   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files        1276     1276           
  Lines       45315    45316    +1     
=======================================
+ Hits        44349    44350    +1     
  Misses        966      966           

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