List label assignments for resource targets

Accepts 1-500 unique resource targets and preserves their request order. Returns exactly one result per target. An eligible target without assignments, a target that does not exist, and a target that the caller cannot access all return an empty labelIds array. The response does not distinguish these cases. labelIds is a set with no meaningful order. The operation is safe and idempotent despite using POST. Eligible targets are owned saved dashboards, visible integration dashboards, readable datasets, visible monitors, and visible notifiers. Each target is checked using its resource's read permission; missing permissions yield empty labelIds for that target. Dashboards shared into the organization are not eligible. A target that uses the wrong identifier for its type returns 400. An unknown target type, duplicate targets, and more than 500 targets fail request validation with 422. An internal failure while resolving targets fails the whole request instead of returning empty label sets.