internxt: persist rotated token returned by the user info call

The refresh endpoint returns a rotated token with a fresh expiry on
every successful call, but getUserInfo discarded it, so routine use
never extended the stored token's life. Once the stored token aged
out, accounts with 2FA enabled could not recover non-interactively
and required a manual reconnect.

Carry the rotated token out of getUserInfo and persist it in NewFs
via the same jwtToOAuth2Token + oauthutil.PutToken path that
refreshJWTToken uses, keeping f.cfg.Token in sync (same pattern as
refreshOrReLogin).

Fixes #9584

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
(cherry picked from commit 66761670da)
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