fix: unwrap optional headers to restore typed content encoding in OpenAPI spec

getParamContentType looked up .meta() on the field schema directly, but
.optional() wraps it in a new node without the annotation. Unwrap
optional/nullable wrappers before looking up the param content meta so
x-pipeline/x-source/x-state headers get their $ref content encoding back.

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

Author: tonyxiao