IAV-1141/integrations): replication, credential validation & Shopify OAuth fix

Adds two new capabilities to the integration layer:

Replication — agents can reuse an existing integration without re-entering credentials. If local persistence fails after orchestration succeeds, the remote integration is rolled back automatically.

Credential Validation — after saving an integration, credentials can be re-validated against the platform API. WhatsApp enforces exactly one WHATSAPP_OAUTH secondary credential.

Also fixes a silent bug in Shopify OAuth where shopSubdomain and appSecretKey were missing from the credential payload sent to n8n.

⚠️ Breaking Changes

• getMainAgentByUserUid header: userUid → x-user-uid
• N8nService and OrchestrationService interfaces have new methods — update all mocks

Related Issues

IAV-1141