List a project's sessions under an idle checker through the scoped SessionIdleCheck query

Drop the ProjectV2 nested idle_checker projection: a project is not an end of the session and idle checker relation, so the project view reads it through the root scopedSessionIdleChecks query from BA-7246 with a checker scope item and a nested session project filter, per the Relations rules in src/ai/backend/manager/api/gql/AGENTS.md. Cover offset and cursor pagination combined with filters. Session IDs selected here feed the exclusion mutation (add one component test for the round trip).

## Success Criteria
- [ ] a project READ user lists sessions under a checker with excluded/effective filters
- [ ] pagination combined with filters works in both offset and cursor modes
- [ ] sessions from other projects never appear
- [ ] returned session ids are accepted by the exclusion mutation (component test)
- [ ] pants test passes for affected packages

JIRA Issue: BA-7282

Read src/ai/backend/manager/api/gql/AGENTS.md and trace the scopedSessionIdleChecks query, checker scope item, nested session project filter, and exclusion mutation. Verify offset and cursor pagination with excluded/effective filters, then add the component round-trip test and run pants test for the affected packages.

Written by the indexing model from the issue text.