fix(core): Send `internal_error` as span status for Vercel error spans

[github-actions]

Note

The pull request "fix(core): Send internal_error as span status for Vercel error spans" was created by @nicohrubec but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Follow up to: #19863

Vercel sets raw error messages as span status. Currently we send them as is, but these values should be normalized to a known SpanStatusType.

I confirmed the behavior with https://github.com/getsentry/testing-ai-sdk-integrations and figured this is straightforward enough to change without an additional test, but let me know if you think I should add one.

[linear-code]

JS-1981