Org and repo levels API for AI Security Detections enablement [Public Preview]

### Value Prop
You can now enable and disable AI Security Detections across your organizations and repositories through the REST API, making it straightforward to roll out the feature at scale. This is especially useful for large enterprises managing hundreds or thousands of repositories, where manual UI configuration is not practical. The API follows the same ergonomics as the code scanning API and respects your existing enablement hierarchy from enterprise down to repository level.

### Expected Outcome
Teams can adopt AI Security Detections faster and more consistently by integrating enablement into their standard infrastructure tooling rather than relying on manual steps in the GitHub UI. This removes a key blocker for automation-first customers and supports broader, more predictable rollouts across complex, multi-org environments.

Start by reviewing the requested REST API enablement behavior at organization and repository levels, and compare it with the existing code scanning API ergonomics. Define the API surface and hierarchy behavior needed to enable and disable AI Security Detections, then confirm that the resulting workflow supports rollout across multiple organizations and repositories.

Written by the indexing model from the issue text.