fix(subscriptions): update existing charge on overridden plan in place (#5549)

[nlenepveu]

Context

Closes #5549.

When a subscription has been updated through the legacy PATCH /subscriptions/{external_id} plan_overrides path and is later updated through the new per-charge PUT /subscriptions/{external_id}/charges/{code} endpoint, the overridden plan ends up holding two charges with the same billable_metric_id and code. Both are enumerated by the canonical fee-creation path (app/services/invoices/calculate_fees_service.rb#create_charges_fees) with no de-duplication, so each event produces two Fee rows. The full source trace and a reproduction sequence on api.eu.getlago.com are in #5549.

Implements option 2 from the issue, per @rsempe's suggestion:

Subscriptions::UpdateOrOverrideChargeService will detect when the resolved charge is already on the overridden plan and update it in place rather than creating a duplicate.

Change

In Subscriptions::UpdateOrOverrideChargeService#find_or_update_charge_override(target_plan), short-circuit when the resolved charge already lives on the overridden plan — update it in place rather than walking up to find_parent_charge and creating a new override on top.

def find_or_update_charge_override(target_plan)
  return update_charge_override(charge) if charge.plan_id == target_plan.id

  parent_charge = find_parent_charge
  existing_override = target_plan.charges.find_by(parent_id: parent_charge.id)

  if existing_override
    update_charge_override(existing_override)
  else
    create_charge_override(parent_charge, target_plan)
  end
end

Behaviour across the three scenarios

Subscription state	Resolved charge.plan_id vs. target_plan.id	Behaviour
Fresh — on the base plan, no overridden plan yet	base_plan.id != overridden_plan.id	Short-circuit doesn't fire. ensure_plan_override creates the overridden plan and populates it via Charges::OverrideService. Existing branch finds the freshly-created override via parent_id match and updates it. Unchanged.
Already overridden via per-charge PUT — parent_id points at base-plan charge	overridden_plan.id == overridden_plan.id	Short-circuit fires → updates the override in place. Same effective behaviour as before, different code path.
Already overridden via legacy plan_overrides PATCH — parent_id: nil, charge sits directly on the overridden plan via Plans::UpdateService#process_charges	overridden_plan.id == overridden_plan.id	Short-circuit fires → updates the legacy charge in place. No second override created. Bug fixed.

Tests

Existing spec contexts in spec/services/subscriptions/update_or_override_charge_service_spec.rb continue to pass; new context "when the charge passed lives directly on the overridden plan with parent_id: nil" covers scenario 3 explicitly:

• Asserts Charge.count does not change (no second override).
• Asserts the existing charge is updated in place (parent_id still nil, same plan_id, override params applied).
• Asserts the overridden plan has exactly one charge for the billable_metric after the call.

Out of scope

• Cleanup of already-affected production subscriptions. That stays a Lago-side concern (DB / support ticket); this fix only prevents new duplicates from forming.
• The full DELETE /subscriptions/{external_id}/charges/{code} endpoint (option 1 from the issue). Left for a follow-up.

cc @rsempe — thanks for the quick triage on #5549.

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