fix(core): Do not overwrite user provided conversation id in Vercel

[github-actions]

Note

The pull request "fix(core): Do not overwrite conversation id if already set" was created by @nicohrubec but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

We set this unconditionally based on the resonse_id so if a user calls Sentry.setConversationId() explicitly this will still be overwritten, which is unexpected.

[linear-code]

JS-1975