feat(billing): scope pending changes to contracts

[volokluev]

Summary

• add target_contract_id to pending-change get/clear APIs
• require target_contract_id on pending-change staging APIs
• expose target_contract_id in get responses for callers/debugging

Test plan

• make build-py
• pre-commit run --files proto/sentry_protos/billing/v1/services/pending_change/v1/endpoint_add_pending_user_config.proto proto/sentry_protos/billing/v1/services/pending_change/v1/endpoint_clear_pending_change.proto proto/sentry_protos/billing/v1/services/pending_change/v1/endpoint_get_pending_change.proto proto/sentry_protos/billing/v1/services/pending_change/v1/endpoint_set_pending_month_interval.proto proto/sentry_protos/billing/v1/services/pending_change/v1/endpoint_set_pending_package.proto

[github-actions]

The latest Buf updates on your PR. Results from workflow ci / buf-checks (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jun 11, 2026, 10:15 PM