POST /api/agent/v2/query

Request body for /v2/construct-query and the fresh-query branch of /v2/query. A single :query key whose value is a JSON object matching [[metabase.lib.schema/external-query]] — the canonical portable MBQL 5 wire format. The query is fully self-describing: the database is derived from the first stage's source-table: or source-card:, all field references are portable FKs ([<db-name>, <schema>, <table-name>, <column-name>]), and there is no auxiliary source_entity / referenced_entities envelope. See resources/metabot/prompts/tools/construct_notebook_query.md for the full format reference (including operators, joins, expressions, multi-stage queries, and FK conventions).

Closed map: any extra top-level keys (notably the legacy source_entity / referenced_entities envelope from before the repr migration) are rejected with a 400 so callers don't silently send fields the server ignores.