test(billing): make fusion_centralize_billing suite hermetic (green baseline)

- test_usage / test_webhook setUp: get-or-create the cpu_seconds metric and
  nexacloud service so the suite no longer collides with existing rows.
- test_invoice_ledger: add _fc_ensure_ca_billing_env (activate CAD + a 13%
  sale tax matching _fc_tax_for) so the ledger tests pass on a clean DB.

Canonical test DB: a FRESH db with l10n_ca installed (a prod clone collides
on fixed-code fixtures across 5 test files). Full suite now exits 0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_centralize_billing/tests/test_usage.py

@@ -9,7 +9,8 @@ class TestRatingCron(TransactionCase):
 
     def setUp(self):
         super().setUp()
-        self.metric = self.env['fusion.billing.metric'].sudo().create(
+        Metric = self.env['fusion.billing.metric'].sudo()
+        self.metric = Metric.search([('code', '=', 'cpu_seconds')], limit=1) or Metric.create(
             {'name': 'CPU seconds', 'code': 'cpu_seconds', 'aggregation': 'sum'})
         self.plan_a = self.env['sale.subscription.plan'].sudo().create(
             {'name': 'Plan A', 'billing_period_value': 1, 'billing_period_unit': 'month'})
@@ -67,7 +68,8 @@ class TestUsageIngestion(TransactionCase):
 
     def setUp(self):
         super().setUp()
-        self.metric = self.env['fusion.billing.metric'].sudo().create(
+        Metric = self.env['fusion.billing.metric'].sudo()
+        self.metric = Metric.search([('code', '=', 'cpu_seconds')], limit=1) or Metric.create(
             {'name': 'CPU seconds', 'code': 'cpu_seconds', 'aggregation': 'sum'})
         self.plan = self.env['sale.subscription.plan'].sudo().create(
             {'name': 'Monthly', 'billing_period_value': 1, 'billing_period_unit': 'month'})