Add shortcuts to AI catalog in global search

What does this MR do and why?

Adds key value pairs so that users can search for Agents or Flows and discover a crosslink to the AI catalog from the global search modal.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/558882

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:global_ai_catalog)

2. Use / to open the search modal
3. Enter Agents and Flows and test the shortcut

MR acceptance checklist

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