Refactor: enforce key alignment with IidPreallocator
## Summary
Resource keys in each `MaxIidsSaver::RESOURCE_QUERIES` (export side) must match the keys in `IidPreallocator::TRACKABLE_RESOURCES` (import side) or preallocation silently breaks. There is currently no enforcement of this — a shared constant should ensure alignment.
### What to do
1. **Add a shared constant** that ensures each `MaxIidsSaver` subclass's `RESOURCE_QUERIES` keys are a subset of `IidPreallocator.trackable_resources.keys`
### Context
This follow-up originates from review comments by `@SamWord` on !228691:
- [Single source of truth for resource keys](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/228691#note_3196158880)
### Relevant files
- `lib/gitlab/import_export/project/max_iids_saver.rb`
- `lib/gitlab/import_export/group/max_iids_saver.rb`
- `lib/gitlab/import/iid_preallocator.rb`
issue