Simplify AI Security Detection enablement by unlinking it from CodeQL Default Setup [Public Preview]

### Value Prop
You can now enable AI security detections independently of CodeQL, so your team gets AI-powered vulnerability detection at the pull request level regardless of which languages your codebase uses. If your projects rely primarily on languages like PHP that aren't covered by CodeQL, you no longer need to configure CodeQL default setup first. This makes it faster and simpler to start catching vulnerabilities with AI on every pull request.

### Expected Outcome
By removing the dependency on CodeQL default setup, AI security detections become accessible to a significantly broader set of development teams, including those working in language ecosystems that CodeQL doesn't cover. This simplifies the enablement experience and reduces the friction that prevented many GitHub Advanced Security customers from using the feature. More teams should be able to activate and benefit from AI-powered security coverage with fewer configuration steps.

The issue names no implementation files, tests, or entry points. Start by locating the AI security detection enablement flow and its dependency on CodeQL default setup; done means teams can enable the detections independently and use them on pull requests without configuring CodeQL first.

索引モデルが issue の本文から書いたものです。