require_managed_files was only checked on upload, so raw provider ids still
reached the batch, fine-tuning and vector store file routes. Ownership rows
exist only for managed ids, so those requests were forwarded under shared
credentials with no tenant check: knowing another tenant's id was enough to
read, run against, cancel or delete their object.
Generalise the file-id guard to validate_managed_id_requirement(resource_id,
resource_kind) and call it on batch create/retrieve/cancel, fine-tuning
create/retrieve/cancel (training_file and validation_file both) and the shared
vector store file id resolver. Behaviour is unchanged when the setting is off.
