Add optional public flag to Collection listing items

[dmitrizagidulin]

Adds an optional boolean public member to each item in the Collections listing (GET /space/{space_id}/collections/), indicating whether the optional PublicCanRead policy is attached to that Collection.

• Surfaced inline so listing consumers don't have to issue one policy probe per item to learn public readability.
• Servers that surface the flag MUST include it on every item (explicit false, never omitted), so clients can distinguish "not public" from "server doesn't surface the flag".
• Fail-closed: unrecognized policy types report as "public": false, consistent with the policy evaluation contract.
• When absent, clients fall back to probing each Collection's policy.

Also updates the listing response example to show the flag.