Update SDK and CLI to the new assignment contract and regenerate schema dumps

Update client/v2/domains_v2/idle_checker_assignment.py and the CLI commands to the checker_id + scope enable/disable contract. Regenerate the GraphQL schema dumps and OpenAPI reference (scripts/generate-graphql-schema.sh; CI auto-commits).

## Success Criteria
- [ ] SDK exposes the new contract; CLI can toggle enabled by checker id and scope
- [ ] schema dumps and openapi.json regenerated with no unexplained diffs
- [ ] pants test passes for affected packages

JIRA Issue: BA-7279

Start with client/v2/domains_v2/idle_checker_assignment.py and locate the CLI commands that handle checker assignments. Run scripts/generate-graphql-schema.sh and inspect the OpenAPI regeneration path, then update the SDK and CLI for checker_id plus scope enable/disable behavior. Done means the SDK and CLI expose the new contract, schema dumps and openapi.json have no unexplained diffs, and pants test passes for affected packages.

Written by the indexing model from the issue text.