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

vitest.config.ts

@@ -5,7 +5,7 @@ export default defineConfig({
   test: {
     globals: true,
     environment: 'node',
-    setupFiles: ['./tests/setup/global-setup.ts'],
+    setupFiles: ['./tests/setup/global-setup.ts', './tests/setup/msw-setup.ts'],
     // Load environment variables from .env.test
     env: {
       NODE_ENV: 'test'