Adds the incoming-request data contracts as Pydantic v2 models. Matches the
MVP spec §3 exactly — fields dropped from the reference spec (use_vision,
reasoning_effort, version, ...) stay out, and `extra="forbid"` catches any
caller that sends them so drift surfaces immediately instead of silently.
Context.files is `list[str | FileRef]`: plain URLs stay str, dict entries
parse as FileRef. This keeps the common case (public URL) one-liner while
still supporting Paperless-style auth headers and per-file size caps.
ix_id stays optional with a docstring warning that callers MUST NOT set it —
the transport layer assigns the 16-char hex handle on insert. The field is
present so `Job` round-trips out of the store.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
