ErrorDetail

Additional structured context. Empty for most errors. On competition-resolution misses and capability gaps the canonical machine-readable list is [`crate::leagues::AVAILABLE_COMPETITIONS`] (`error.available_competitions`): every registered slug on `NOT_FOUND` / invalid-competition `BAD_REQUEST`, or the slugs that implement the named capability on `NOT_SUPPORTED`. The engine registry is the only source of truth — clients (including MCP) pass this array through and must not maintain a parallel list. The human `message` may summarize. Value type: object.