Follow-up from "Add query for hybrid_similarity"

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The following discussion from !169091 (merged) should be addressed:

• @maddievn started a discussion: (+2 comments)

  I read through the AI docs and there have been some changes since the issue hybrid search was implemented. We need to update our code to comply with the policies for AI features. It seems we need to register a feature in AiFeaturesCatalogue, use ::Gitlab::Llm::FeatureAuthorizer.new(container: subject_container, feature_name: action_name).allowed? and add a policy. We should work with #g_ai_framework to make sure we're getting it right.

  What we want at the end is: a beta feature that uses the semantic_search_issue unit primitive to check if the project has AI features enabled and the user has AI features enabled. Right now it's limited to .com with the Gitlab::Saas.feature_available?(:ai_vertex_embeddings) but this should be removed as well.

Also !169091 (comment 2160749398)