Add licensedFeatureAvailability GraphQL field to Project, Namespace, and Group

What does this MR do and why?

Adds a new licensedFeatureAvailability GraphQL field to Project, Namespace, and Group types, returning whether a given licensed feature is available and the minimum plan required if not. This provides the backend API needed for Duo agentic chat to surface clear tier-gating messages to users.

Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/588337.

Related AI Gateway MR: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!5089 (merged).

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/588337

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start GDK with an EE license enabled.
2. Test on a project:

query {
  project(fullPath: "gitlab-org/gitlab-shell") {
    licensedFeatureAvailability(feature: SECURITY_DASHBOARD) {
      available
      requiredPlan
    }
  }
}

3. Test on a group:

query {
  group(fullPath: "gitlab-org") {
    licensedFeatureAvailability(feature: EPICS) {
      available
      requiredPlan
    }
  }
}

4. Both queries should return available: true and requiredPlan: null when the feature is licensed. On a free-tier namespace, expect available: false with the minimum required plan (e.g., "premium" or "ultimate").

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