fix(server): stop sending x-vercel-ip

[feugy]

🖖 What's in there?

@vercel/analytics/server stops sending x-vercel-ip on the beacon it emits from the customer's function. Ingestion no longer reads it (vercel/api#95447), so the header is dead weight.

Server-side events are attributed to the IP that reaches ingestion. Before and after this change, that is the function's egress, not the visitor's browser: the header copied the function's incoming x-forwarded-for, which on Vercel carries the request's edge hop rather than the visitor. Tinybird confirms it over 7 days: 18 of the 23 owners with ≥10K server-side events have a single country (their function region), and vercel.com's own server-side events show as 76% DE / 17% US.

	Before	Now
Headers sent	user-agent, x-vercel-ip, x-va-server, cookie	user-agent, x-va-server, cookie
Country shown for server-side events	function region, via a forgeable header	function region, via the connection

🤺 How to test?

pnpm --filter @vercel/analytics test — server tests updated to assert the header is gone. No change to the event body or to any public API.

🔬 Notes to reviewers

• x-vercel-ip was never documented (vercel.com/docs or this repo). It was a private contract with api-analytics-ingestion, which was accepting any client-supplied IP on every beacon, including pageviews and vitals from the browser.
• Reading x-real-ip instead would give the visitor IP for direct traffic, but it would still be a client-supplied value that ingestion must take on faith, and the edge for anyone behind Cloudflare. Not worth keeping a forgeable channel for it.
• Worth a line in the docs for track() from @vercel/analytics/server: server-side events are attributed to the region of the function that sent them.

🔗 Related PRs

• vercel/api#95447 — ingestion stops reading x-vercel-ip and other client-supplied IP headers
• vercel/proxy#25782 — proxy forwards the verified visitor IP for browser beacons behind a reverse proxy

[vercel]

Your Vercel team Analytics Test Projects is not permitted to deploy from this git repository. Contact an administrator to add github organization vercel as a Protected Git Scope in Analytics Test Projects on Vercel. Once added, commit again to see your changes.

Learn more: https://vercel.com/docs/security/protected-git-scopes

[feugy]

agent slop, we don't need this