Fix `verification user not found` error during identity verification

What does this MR do and why?

• Partially resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/347
• We haven't been able to find out all the causes of this error, but one scenario that can occur is when a user completes identity verification and then uses their browser's back button to navigate back to the identity verification page. In that case, the Verification user not found error does get logged.
• To resolve that we check if the user is already logged-in/verified in the require_verification_user and we add logging events to all methods so we can find out other triggers for this error.

Screenshots or screen recordings

Before	After
before-verification-user-not-found	after-verification-user-not-found

How to set up and validate locally

Setting-up Identity Verification

1. In rails console, enable the feature flag and configure identity verification

> Feature.enable(:identity_verification)
> ApplicationSetting.first.update(email_confirmation_setting: "hard", require_admin_approval_after_user_signup: false)

1. Sign-up as a new user on /users/sign_up
2. You should see a page asking you to confirm your email address.
3. Go to /rails/letter_opener and complete the email verification & onboarding process.
4. Now, using the browser's back button navigate back to the identity verification page. You should see the Verification successful page and you won't be able to find any logs in application_json.log with error message Verification user not found.

MR acceptance checklist

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