feat: Add invariant check for rental subnets

[dsarlis]

This PR adds an invariant for the subnet record in the registry regarding rental subnets, specifically a check is added that only application subnets can be rented, i.e. have a "free" cycles cost schedule.

[github-actions]

This pull request changes code owned by the Governance team. Therefore, make sure that
you have considered the following (for Governance-owned code):

1. Update unreleased_changelog.md (if there are behavior changes, even if they are
   non-breaking).

2. Are there BREAKING changes?

3. Is a data migration needed?

4. Security review?

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Reference(s)

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GOVERNANCE_CHECKLIST_REMINDER_DEDUP

[dsarlis]

This is a code hardening change and it should hold in the current registry state. Therefore, there are no breaking changes by adding it as the check should succeed.

[github-actions]

Run on 3f3f8d1 URL: https://github.com/dfinity/ic/actions/runs/21872425435

[github-actions]

Run on 53d1e65 URL: https://github.com/dfinity/ic/actions/runs/21901940017