Draft: Drive the Policy Store editor from the URL

What does this MR do and why?

Gives the Policy Store editor URL-addressable state, so a refresh or a shared link restores the exact editor view instead of falling back to the list.

• no params — the policy list
• ?editor=new&step=build|scope|review — creating a policy
• ?editor=edit&policy=<id-or-name>&step=… — editing a policy, addressed by id when it has one, by name otherwise

Guard rails: an unknown step normalizes to build; an unknown editor value reads as the list; an edit link to a policy the list does not know returns to the list.

Implementation — plain frontend routing, no Vue Router and no backend changes (query params survive a refresh on the existing index route):

• editor_url.js owns the URL shape over ~/lib/utils/url_utility (queryToObject, setUrlParams, removeParams).
• App holds the state parsed from the URL, navigates with updateHistory, and follows popstate, so browser Back/Forward walk the wizard.
• StepWizard takes the step as a prop and emits change-step; Next/Back/Edit all round-trip through the URL.

Stacked on !248607 (merged) (targets 607341-policy-store-editor-design); merge that first.

References

• Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/607341
• Epic: https://gitlab.com/groups/gitlab-org/-/epics/22027
• Predecessor MR: !248607 (merged)

Screenshots or screen recordings

No visual changes — the list and editor render exactly as before; only the URL bar changes while navigating.

Before	After
/-/security/policy_store for every editor state	/-/security/policy_store?editor=new&step=build, …?editor=edit&policy=1&step=review, …

How to set up and validate locally

1. Enable the experiment for a top-level group:

   # rails console
   Feature.enable(:security_policies_v2)
   ApplicationSetting.current.update!(policy_store_experiment_enabled: true)
   Group.find_by_full_path('flightjs').namespace_settings.update!(policy_store_experiment_enabled: true)

2. Visit http://gdk.test:3000/groups/flightjs/-/security/policy_store.

3. Click Create new policy — the URL gains ?editor=new&step=build; Next/Back move it through scope and review, and browser Back/Forward walk the steps.

4. Refresh on any step — the editor reopens on that step.

5. Edit a policy from the list — the URL gains ?editor=edit&policy=1&step=build; refresh restores it. Replace 1 with the policy name (URL-encoded) — the same editor opens.

6. Try a bogus step (…&step=nonsense) or an unknown policy (…&policy=999) — you land on build / the list respectively.

MR acceptance checklist

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