Mechanical signature updates for LinkAttachmentsToMessage callsites, plus:
db-level OwnershipGuard test (owned links, foreign never links, legacy
NULL-uploader claimable, nonexistent skipped) and a service-level
SendMessage test proving skip semantics end-to-end including the
already-linked retry path.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>