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.

src/test/accounts.test.ts

@@ -0,0 +1,56 @@
+import { describe, it, expect } from "vitest";
+import { GET as listAccounts } from "../pages/api/accounts/index";
+import { GET as getAccount } from "../pages/api/accounts/[id]/index";
+import { GET as listTransactions } from "../pages/api/accounts/[id]/transactions/index";
+
+describe("Accounts API", () => {
+  describe("GET /api/accounts", () => {
+    it("should return all accounts", async () => {
+      const response = await listAccounts();
+      const accounts = await response.json();
+
+      expect(response.status).toBe(200);
+      expect(accounts).toHaveLength(2);
+      expect(accounts[0]).toHaveProperty("id", "1");
+      expect(accounts[1]).toHaveProperty("id", "2");
+    });
+  });
+
+  describe("GET /api/accounts/:id", () => {
+    it("should return a specific account", async () => {
+      const response = await getAccount({ params: { id: "1" } });
+      const account = await response.json();
+
+      expect(response.status).toBe(200);
+      expect(account).toHaveProperty("id", "1");
+      expect(account).toHaveProperty("name", "Test Checking");
+    });
+
+    it("should return 404 for non-existent account", async () => {
+      const response = await getAccount({ params: { id: "999" } });
+      const error = await response.json();
+
+      expect(response.status).toBe(404);
+      expect(error).toHaveProperty("error", "Account not found");
+    });
+  });
+
+  describe("GET /api/accounts/:id/transactions", () => {
+    it("should return transactions for a specific account", async () => {
+      const response = await listTransactions({ params: { id: "1" } });
+      const transactions = await response.json();
+
+      expect(response.status).toBe(200);
+      expect(transactions).toHaveLength(1);
+      expect(transactions[0]).toHaveProperty("accountId", "1");
+    });
+
+    it("should return empty array for account with no transactions", async () => {
+      const response = await listTransactions({ params: { id: "999" } });
+      const transactions = await response.json();
+
+      expect(response.status).toBe(200);
+      expect(transactions).toHaveLength(0);
+    });
+  });
+});