Fix PhoneVerification::TelesignCallbacksController#notify returning 500s

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/520

Problem

The action errors out because of the use of find_by inside a scope. find_by doesn't return an ActiveRecord::Relation so when it eventually returns nil (i.e. when it can't find a record) the enclosing scope returns all (ActiveRecord::Relation) (source) instead of nil.

    def track_status_update_event
      # when there is no matching record for `payload.reference_id` the scope returns `all` instead of `nil`
      record = Users::PhoneNumberValidation.by_reference_id(payload.reference_id)

      # when find_by returns `nil` and the scope returns `all` as a result, the early return is skipped
      return unless record

      event = payload.failed_delivery? ? 'telesign_sms_delivery_failed' : 'telesign_sms_delivery_success'

      Gitlab::Tracking.event(
        'IdentityVerification::Phone',
        event,
        user: record.user, # this throws the "undefined method `user' for #<ActiveRecord::Relation"
        extra: { country_code: record.country, status: payload.status }
      )
    end

Solution

This MR moves the use of find_by from a scope (.by_reference_id) to a class method. The new class method (same name) returns either nil or a Users::PhoneNumberValidation as expected.

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