Raise the privileged large-PDF cap to the 256MB architectural ceiling (#4437)

The privileged by-reference cap was 200MB while every other layer of the
pipeline is already sized for 256MB: largePdfLimitBytes clamps to the
FIRE_PDF_BY_REFERENCE_MAX_FILE_SIZE ceiling, and the downstream PDF
service accepts 256MB GCS inputs. Raising the default closes the gap so
allowlisted teams can process documents in the 200-256MB range.

Co-authored-by: Abimael Martell <7519471+abimaelmartell@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>