fix: append intl headers in with-next-intl instead of overwrite

[tusharpandey13]

Fixes #2340

Use Headers.append() instead of Headers.set()

See Headers: append() method

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.61%. Comparing base (df1725e) to head (b67390c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2386   +/-   ##
=======================================
  Coverage   87.61%   87.61%           
=======================================
  Files          37       37           
  Lines        4086     4086           
  Branches      809      809           
=======================================
  Hits         3580     3580           
  Misses        503      503           
  Partials        3        3           

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[tusharpandey13]

@claude please review this PR

[github-actions]

Claude finished @tusharpandey13's task —— View job

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PR Review Complete

Overall Assessment

✅ This PR correctly fixes a critical bug where set-cookie headers were being overwritten instead of preserved. The change from Headers.set() to Headers.append() is the appropriate solution for handling multi-value headers like set-cookie. The code quality is excellent with clear documentation explaining the rationale, and the fix aligns with HTTP standards for header handling.

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Job run | Branch: bugfix/sample-copy-cookie-headers