[vercel] fix: return 3xx responses when redirect is manual in client.fetch() (#15953)

## Summary

- `client.fetch()` now returns 3xx responses directly when `redirect:
'manual'` is passed, instead of entering the error/retry path
- This is opt-in — no existing behavior changes since no code currently
passes `redirect: 'manual'` through `client.fetch()`

## Why
This enables CLI commands that need to extract `Location` headers from
redirect-based API endpoints (e.g., endpoints that initialize
server-side state and redirect to a browser URL).

Today, the 3xx response hits `!res.ok` (302 is outside 200-299), doesn't
match the 400-499 bail condition, and falls through to `throw error` —
causing infinite retries.



## Test plan

- [x] New test: 302 with `redirect: 'manual'` returns raw response with
Location header
- [x] New test: 302 without `redirect: 'manual'` follows the redirect
(default behavior unchanged)
- [x] Existing proxy test still passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>