Enable DAP access for free-tier namespaces

What does this MR do and why?

Enables Duo Agent Platform (DAP) access for free-tier SaaS namespaces that have an active gitlab_credits add-on.

This is the backend foundation for the feature. It introduces:

• A dap_free_tier_gitlab_credits feature flag (default: disabled) to safely roll out access
• Namespace#has_active_gitlab_credits_add_on? checks for an active gitlab_credits purchase, gated behind the flag
• licensed_ai_features_available? on Group and Project, extended to return true for gitlab_credits namespaces on SaaS
• StageCheck, llows credits-eligible DAP features to bypass the license check for gitlab_credits namespaces
• Ai::Catalog#duo_agent_platform_enabled_for_user? falls back to a credits-based check when the user has no Premium+ group
• UserPreference#duo_default_namespace_candidates — includes gitlab_credits purchases alongside duo_core for namespace selection

MR series

This change cannot be fully tested in isolation. End-to-end behaviour is only visible once all parts are merged.

• !226462 (merged) - Backend core access gate ⬅️ you are here
• !226470 (merged) - Gating, policies, and catalog finder
• !226475 (merged) - Frontend and settings helpers

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

This MR can't be tested in isolation, a full working PoC is available in this MR

MR acceptance checklist

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