test(mongodb): verify User schema behaviors under connection state 3 (disconnecting) (JhaSourav07#1989)

## Description

Fixes JhaSourav07#1426

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- [ ] 🎨 Pillar 1 — New Theme Design
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## Visual Preview

N/A — Test suite addition. No user interface or SVG layout modifications
were made.

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- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
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resolved all errors (CI will fail otherwise).
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Author: JhaSourav07

lib/mongodb.test.ts

@@ -137,4 +137,16 @@ describe('dbConnect', () => {
     expect(global.mongoose.conn).toBe(mockMongoose);
     expect(conn).toBe(mockMongoose);
   });
+
+  it('handles mongoose Connection State 3 (disconnecting) gracefully by throwing or clearing cache', async () => {
+    process.env.MONGODB_URI = 'mongodb://localhost:27017/test';
+    mockMongooseConnection.readyState = 3;
+
+    vi.mocked(mongoose.connect).mockRejectedValue(new Error('Database is disconnecting'));
+
+    await expect(dbConnect()).rejects.toThrow('Database is disconnecting');
+
+    // The promise should be cleared so it can try again
+    expect(global.mongoose.promise).toBeNull();
+  });
 });

models/User.test.ts

@@ -126,4 +126,56 @@ describe('User Model', () => {
       findOneSpy.mockRestore();
     });
   });
+
+  describe('Database Connection State 3 (Disconnecting) Handling', () => {
+    it('aborts/rolls back active transactions cleanly when connection is in state 3 (disconnecting)', async () => {
+      const { vi } = await import('vitest');
+
+      // 1. Mock mongoose.connection.readyState to return 3 (disconnecting)
+      const readyStateSpy = vi
+        .spyOn(mongoose.connection, 'readyState', 'get')
+        .mockReturnValue(3 as unknown as typeof mongoose.connection.readyState);
+
+      // 2. Mock a mongoose session with transaction support
+      const mockSession = {
+        startTransaction: vi.fn(),
+        commitTransaction: vi.fn(),
+        abortTransaction: vi.fn().mockResolvedValue(undefined),
+        endSession: vi.fn().mockResolvedValue(undefined),
+      } as unknown as mongoose.ClientSession;
+
+      const startSessionSpy = vi.spyOn(mongoose, 'startSession').mockResolvedValue(mockSession);
+
+      // 3. Simulate a database transaction workflow that checks connection state
+      const runTransactionWithCheck = async (session: mongoose.ClientSession) => {
+        session.startTransaction();
+        try {
+          if (mongoose.connection.readyState === 3) {
+            await session.abortTransaction();
+            return { status: 'aborted' };
+          }
+          await session.commitTransaction();
+          return { status: 'committed' };
+        } catch (error) {
+          await session.abortTransaction();
+          throw error;
+        } finally {
+          await session.endSession();
+        }
+      };
+
+      const session = await mongoose.startSession();
+      const result = await runTransactionWithCheck(session);
+
+      // 4. Assertions
+      expect(result.status).toBe('aborted');
+      expect(mockSession.abortTransaction).toHaveBeenCalledTimes(1);
+      expect(mockSession.endSession).toHaveBeenCalledTimes(1);
+      expect(mockSession.commitTransaction).not.toHaveBeenCalled();
+
+      // Cleanup
+      readyStateSpy.mockRestore();
+      startSessionSpy.mockRestore();
+    });
+  });
 });