Summary

Makes AxiosHeaders#getSetCookie() return an array for both raw parsed headers and programmatic single-value headers.

Linked issue

N/A

Changes

• Wrap a single set-cookie value in an array.
• Keep existing array and missing-header behavior unchanged.
• Add a regression test for programmatic headers.set("set-cookie", ...).

Validation

• npm run test:vitest:unit -- tests/unit/axiosHeaders.test.js
• npm run lint -- lib/core/AxiosHeaders.js
• git diff --check

Checklist

• Tests added or updated (or N/A with reason)
• Docs/types updated if public API changed (index.d.ts and index.d.cts)
• No breaking changes (or called out explicitly above)

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