Limit the number of descendant policies

What does this MR do and why?

This MR limits the number of descendant security policy configurations loaded to prevent timeouts when dealing with large groups. The change introduces a hard limit of 50 descendant policy projects to improve performance of security policies API.

Currently, policies are read directly from YAML rather than the database, which prevents pagination. This temporary limit prevents timeouts while we work toward a permanent solution. We plan to migrate policy retrieval from the database and implement proper pagination in [Backend]: Replace Gitaly request with DB for ... (#541270). Once that's complete, this limit can be removed in favor of paginated results.

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Related to #520440