feat: support pipeline/state config in JSON request body (#291)

Add an alternative to passing config via x-pipeline/x-state headers:
callers can now send Content-Type: application/json with a body like
{pipeline, state?, body?} containing native JSON objects (no string
escaping). The content-type header determines which mode is used —
application/json for the new path, anything else for the existing
header + NDJSON behavior. Purely additive, fully backward compatible.

Made-with: Cursor
Committed-By-Agent: cursor
Made-with: Cursor
Committed-By-Agent: cursor

Author: tonyxiao