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_webhook.py

@@ -13,11 +13,17 @@ class TestWebhookEngine(TransactionCase):
 
     def setUp(self):
         super().setUp()
-        self.service = self.env['fusion.billing.service'].sudo().create({
+        Service = self.env['fusion.billing.service'].sudo()
+        vals = {
             'name': 'NexaCloud', 'code': 'nexacloud',
             'webhook_url': 'https://api.vps.nexasystems.ca/billing/webhook',
             'webhook_secret': 'whsec_test',
-        })
+        }
+        self.service = Service.search([('code', '=', 'nexacloud')], limit=1)
+        if self.service:
+            self.service.write(vals)
+        else:
+            self.service = Service.create(vals)
         self.Webhook = self.env['fusion.billing.webhook'].sudo()
 
     def test_enqueue_signs_payload(self):