Add feature flag and routing for new iterable leads endpoint

Summary

Add feature flag and routing logic on GitLab.com to support the new leads/gitlab_com/iterables endpoint on CustomersDot.

Problem

GitLab.com currently routes iterable lead creation to the legacy trials_controller.rb endpoint on CustomersDot. We need to add a feature flag to gradually migrate to the new dedicated endpoint.

Implementation Details

Feature Flag

Name: new_iterable_lead_endpoint

Type: gitlab_com_derisk (percentage-based rollout)

• Create a feature flag rollout issue following https://docs.gitlab.com/development/feature_flags/#create-a-new-feature-flag.
• Add the issue to the current epic.

Code Changes

1. Locate iterable lead creation code:

• Search for calls to CustomersDot iterable endpoint
• Likely in a service, helper, or controller (search for iterable or create_iterable)

2. Add conditional routing:

def iterable_lead_url
  if Feature.enabled?(:new_iterable_lead_endpoint)
    "#{subscription_portal_url}/leads/gitlab_com/iterables"
  else
    "#{subscription_portal_url}/trials/iterable" # Legacy endpoint
  end
end

Rollout Plan

Actual rollout will be handled in a separate rollout issue following GitLab's feature flag rollout process. This issue only creates the infrastructure.