fix(api): resolve header overwriting and delimiter issue in Access-Control-Expose-Headers

[gacevicljubisa]

…response header

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

• Replaced w.Header().Set with w.Header().Add to append headers rather than overwrite them.
• Corrected delimiter for Access-Control-Expose-Headers from ; to , as per MDN documentation.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

• Browser behavior: The semicolon delimiter (;) was causing issues, particularly in Chromium-based browsers. This was corrected by switching to a comma (,) delimiter for separating multiple header values.
• Header overwriting: The use of w.Header().Set was causing overwriting of Access-Control-Expose-Headers. Replacing it with w.Header().Add ensures headers are appended, resolving inconsistencies in the response headers.

Related Issue (Optional)

[2.4.0] Access-Control-Expose-Headers is overridden, breaking browser Swarm apps #4959

Screenshots (if appropriate):

[Cafe137]

Would it be possible to add tests that check that headers are added instead of being replaced, to avoid a future regression?

[gacevicljubisa]

Would it be possible to add tests that check that headers are added instead of being replaced, to avoid a future regression?

Yes, sure.

[janos]

LGTM. Thanks for addressing my comments.