Add SessionV2.idleChecks as a SessionIdleCheck relation type connection

Add an idleChecks connection to SessionV2 over the SessionIdleCheck relation type from BA-7245, on the connection base from BA-7887. The relation type is not a Node: the checker is a Node field resolved through the idle checker bulk loader, and relation values (sources, excluded, effective, expire_at, last_status) are plain fields. Filter and order are SessionIdleCheck's own and nest checker conditions. The cursor encodes the (session_id, idle_checker_id) primary key. Disabled sources and excluded pairs are returned too. Authorization is session READ; only the safe checker projection is exposed. added_version NEXT_RELEASE_VERSION; regenerate schema dumps.

## Success Criteria
- [ ] session with a checker applied through two scopes: one row with two sources
- [ ] excluded pair shows excluded=true, effective=false
- [ ] session READ suffices; no global checker permission needed
- [ ] cursor pagination works without an id field
- [ ] schema dumps regenerated
- [ ] pants test passes for affected packages

JIRA Issue: BA-7281

Start by locating the SessionV2 and SessionIdleCheck definitions and the existing connection base from BA-7887. Trace how the checker bulk loader, authorization, filtering, ordering, and cursor pagination are represented, then run the pants tests for the affected packages. Done means all listed success criteria pass and the schema dumps are regenerated.

Written by the indexing model from the issue text.