Delegate identity verification marketing panel to registration type

What does this MR do and why?

Extract the identity-verification right-panel image and heading into a delegated abstraction on the onboarding registration types, enabling per-flow customisation without view-level branching.

This is the infrastructure / shared abstraction half of the Unified Registration: Trial and Unified Registration: Free work. The per-flow feature-flag wiring lands in the follow-up MRs:

• Free flow: !237935 (merged) (wires free_registration_unification)
• Trial flow: !237932 (merged) (wires trial_unification)

Architecture

• New mixin Onboarding::IdentityVerificationPanelDefaults provides the default subscription/pipeline image + "Ship software faster" heading.
• TrialRegistration, InviteRegistration, SubscriptionRegistration, and SubscriptionSmRegistration extend the mixin.
• FreeRegistration defines its own overrides returning subscription/collab + "Collaborate and accelerate in one place".
• EE::Onboarding::StatusPresenter delegates two new methods (identity_verification_panel_image / _heading) to the resolved registration type.
• registrations_identity_verification/{show,success}.html.haml now populate the registration_marketing_panel_* content_for slots from the presenter delegation instead of hardcoded values.

Visible behaviour change

For the SM subscription flow with subscription_sm_unification already enabled today:

• show.html.haml → unchanged (subscription/pipeline + "Ship software faster" both before and after — SubscriptionSmRegistration inherits the mixin default which matches the previous hardcoded value).
• success.html.haml → changes from subscription/collab + "Collaborate and accelerate in one place" (hardcoded) to subscription/pipeline + "Ship software faster" (via the mixin default). This aligns the success-page illustration with the rest of the SM unified flow (the SM design uses pipeline throughout).

The free, invite, and non-unified subscription flows are unchanged because the layout dispatch (registration_layout → 'registration_two_panel' only when unification_enabled?) keeps the right panel from rendering, so the new delegation values are computed but never shown.

The trial flow has no visible change in this MR because TrialRegistration.unification_enabled? still returns false until !237932 (merged) wires the FF.

References

• Trial epic: gitlab-org&21601
• Free epic: gitlab-org&21602
• Free follow-up: !237935 (merged)
• Trial follow-up: !237932 (merged)

How to set up and validate locally

The only flow with a user-visible change in this MR is SM subscription with subscription_sm_unification enabled on the post-verification success screen.

1. Ensure subscription_sm_unification is enabled and the onboarding SaaS feature is on:

   # rails console
   Feature.enable(:subscription_sm_unification)

2. Sign out (or use a fresh browser profile so you're starting unauthenticated).

3. Visit the SM subscription purchase URL to seed the stored user location with the SM deployment type:

   https://gdk.test:3443/-/subscriptions/new?deployment_type=self_managed&plan_id=2c92a01176f0d50a0176f3043c4d4a53

4. Complete the registration form and proceed to email verification (use http://gdk.test:3000/rails/letter_opener/ to grab the verification code).

5. After successful verification, on the /users/sign_up/successful_verification interstitial, verify the right panel shows:

   • Image: the pipeline illustration (subscription/pipeline.png — app/assets/images/subscription/pipeline.png)
   • Heading: "Ship software faster"

   Previously (master) this would have shown the collab illustration + "Collaborate and accelerate in one place".

6. (Sanity check, no change expected) Repeat with subscription_sm_unification disabled (Feature.disable(:subscription_sm_unification)). The flow falls back to the 'devise' layout and no right panel is shown — there is nothing to verify visually because the marketing panel doesn't render.

7. (Sanity check, no change expected) Run any free / invite / non-unified subscription sign-up. The right panel does not render (their unification_enabled? returns false), so the delegation result is computed but never shown.

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