Validate a pipeline definition

Validate a pipeline definition without deploying it. Accepts the same body shape as POST /pipelines, so a request that passes validation here can be submitted to POST /pipelines unchanged. name may be provided at the top level or inside definition.name (top-level wins if both are present); a missing, malformed, or reserved name is reported as a { field: "name" } entry in errors rather than rejected as a bad request.