Add the ProjectV2 idle checker sessions nested connection

[seedspirit]

On ProjectV2, add an idle_checker(checker_id) scoped projection with a sessions(filter, pagination) nested connection following the users/usage_buckets pattern, backed by the checker-to-sessions search path. Offset and cursor pagination; project READ gate. Session IDs selected here feed the exclusion mutation (add one component test for the round trip). The resource-group node is out of scope. added_version 26.9.0; regenerate schema dumps.

Success Criteria

• 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