Expand First Party Parent Child Relations

expand[] is implemented and grant-safe, but only a small set of first-party parent-child relations are enabled. Assistants still have to do N+1 reads for common records such as Slack messages with attachments/reactions or other safe child collections.

The reference expansion engine supports one-hop parent-to-child expansion:

First-party connector manifests SHALL enable query.expand only for relations that the current reference engine can serve as one-hop parent-to-child expansions with child grant projection.