feat(tests): add unit tests for accounts and transactions APIs

- Updated package.json to include Vitest for testing and added necessary devDependencies.
- Created accounts.test.ts to test the accounts API endpoints for listing and retrieving accounts.
- Implemented setup.ts to reset test data before each test run.
- Developed transactions.test.ts to cover creating, updating, and deleting transactions through the API.
- Added vitest.config.ts for configuring Vitest with appropriate settings and coverage options.

vitest.config.ts

@@ -0,0 +1,21 @@
+/// <reference types="vitest" />
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    // Increase timeout for slower CI environments
+    testTimeout: 10000,
+    // Use the setup file we created
+    setupFiles: ["./src/test/setup.ts"],
+    // Ensure we're using the right environment
+    environment: "node",
+    // Only include test files
+    include: ["src/test/**/*.{test,spec}.{ts,js}"],
+    // Configure coverage collection
+    coverage: {
+      provider: "v8",
+      reporter: ["text", "json", "html"],
+      exclude: ["node_modules/", "src/test/**/*", "**/*.d.ts"],
+    },
+  },
+});