fix: map reasoning-delta to reasoning_content instead of content (#11847)

The Vercel stream converter was incorrectly mapping reasoning-delta
events to delta.content, causing reasoning/thinking content to be
treated as regular assistant content. This fix maps it to
delta.reasoning_content so downstream consumers can properly
distinguish reasoning from regular output.

Fixes #11069

Co-authored-by: MumuTW <clothl47364@gmail.com>