Key under which a MiddlewareFunction carries its RouteValidationMetadata.

Created with Symbol.for, not Symbol(), so two copies of this package in one process resolve the same key — the SECURITY_METADATA precedent, and for the same reason: a locally-created symbol would simply miss on every read, silently.

Prefer withValidationMetadata and validationMetadataOf over touching this directly; the symbol is exported so a validating middleware outside @setu-ts/validation-plugin can be branded too.