test: add Phase 4 database integration tests (partial)

- Add comprehensive test utilities for database testing
- Implement connection management tests for in-memory and file databases
- Add transaction tests including nested transactions and savepoints
- Test database lifecycle, error handling, and performance
- Include tests for WAL mode, connection pooling, and constraints

Part of Phase 4: Integration Testing

tests/mocks/n8n-api/data/credentials.ts

@@ -0,0 +1,49 @@
+/**
+ * Mock credential data for MSW handlers
+ */
+
+export interface MockCredential {
+  id: string;
+  name: string;
+  type: string;
+  data?: Record<string, any>; // Usually encrypted in real n8n
+  createdAt: string;
+  updatedAt: string;
+}
+
+export const mockCredentials: MockCredential[] = [
+  {
+    id: 'cred_1',
+    name: 'Slack Account',
+    type: 'slackApi',
+    createdAt: '2024-01-01T00:00:00.000Z',
+    updatedAt: '2024-01-01T00:00:00.000Z'
+  },
+  {
+    id: 'cred_2',
+    name: 'HTTP Header Auth',
+    type: 'httpHeaderAuth',
+    createdAt: '2024-01-01T00:00:00.000Z',
+    updatedAt: '2024-01-01T00:00:00.000Z'
+  },
+  {
+    id: 'cred_3',
+    name: 'OpenAI API',
+    type: 'openAiApi',
+    createdAt: '2024-01-01T00:00:00.000Z',
+    updatedAt: '2024-01-01T00:00:00.000Z'
+  }
+];
+
+/**
+ * Factory for creating mock credentials
+ */
+export const credentialFactory = {
+  create: (type: string, name?: string): MockCredential => ({
+    id: `cred_${Date.now()}`,
+    name: name || `${type} Credential`,
+    type,
+    createdAt: new Date().toISOString(),
+    updatedAt: new Date().toISOString()
+  })
+};