Add Policy Store Build policy step

What does this MR do and why?

Summary

This merge request introduces a new "Build Policy" step to the policy editor wizard, allowing users to visually assemble a policy by selecting triggers, rules, and actions from a categorized catalog — rather than writing configuration manually.

Key changes include:

• New UI components: A BuilderSection displays the user's current selections (with expand/collapse and remove buttons), and a CatalogGroup shows available options in a searchable, grouped list with selection indicators.
• New serializer: Translates between the editor's internal form state and the format the Policy Store API expects, handling both saving and loading of existing policies.
• Wizard integration: The new Build step is inserted into the existing step-by-step wizard flow, and the overall policy summary (used in the review step) now includes the trigger, rules, and actions chosen in this step.
• Code cleanup: Some verbose comments in the category constants file were shortened, and a few minor refactors were made to the wizard's step label logic.

In plain terms: users can now pick the building blocks of their policy (when it runs, what it checks, and what it does) through a point-and-click interface, instead of editing raw configuration.

A3's own changes

Screenshots or screen recordings

Description	UI
Policy builder

References

• Plan: https://gitlab.com/gitlab-org/gitlab/-/issues/607341
• Depends on: !247890 (merged) (A2, included in this branch)
• Builds on: !247679 (merged) (A1, merged)
• Ported shell: !246660 (closed)

How to set up and validate locally

The Policy Store is behind a flag and a two-level experiment opt-in:

1. Enable the feature flag:

   Feature.enable(:security_policies_v2)

2. Enable the instance setting:

   ApplicationSetting.current.update!(policy_store_experiment_enabled: true)

3. Enable it for a top-level group with an Ultimate licence:

   Group.find_by_full_path('your-group').namespace_settings.update!(policy_store_experiment_enabled: true)

4. Go to Group → Secure → Policy Store, then New policy.

5. On Build policy: pick the Deployment trigger, then a rule or two, then an action. Check that

   • selecting a second trigger replaces the first
   • each selected entry expands into its own config form
   • the Rego rule is pre-filled with the deployment gate template and Browse templates works
   • values survive clicking Next to Scope and Back again

yarn jest ee/spec/frontend/policy_store

126 tests across 12 suites. The serializer's round-trip spec (state → payload → state) is the one to look at first — it is the contract C2 depends on.

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