Validate Document

Validate PDF and return field detection results.

Unauthenticated on purpose (/docs/api documents it as such): this is the dry run an agent performs to check its field syntax before spending a credit on POST /api/v1/documents/send, and the send dialog uses it to read the document's parties so the sender never has to guess a role. Being auth-free is also why the budgets and the PDFValidator call are not optional -- without them, anyone can hand this route arbitrary bytes and have them parsed.