test(models): verify createdAt default is callable and returns timestamp (JhaSourav07#1116)

## Description
Added a unit test to ensure the createdAt filed on the users model uses
a callable default(i..Date.now)not a static timestamp.
The test checks:
the schema default is function and calling that function returns a
numeric, finite timestamp.

Why:
Prevents a regression where Date.now() might be used at schema compile
time (creating a fixed timestamp for all new documents) instead of
Date.now as a callable.


Fixes JhaSourav07#1108 

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview
N/A

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- [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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`feat(themes): ...`, `fix(calculate): ...`).
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parameter.
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Author: JhaSourav07

models/User.test.ts

@@ -16,6 +16,21 @@ describe('User Model', () => {
       expect(usernamePath.options.lowercase).toBe(true);
     });
 
+    describe('createdAt schema', () => {
+      it('uses a callable default that returns a timestamp', () => {
+        const createdAtPath = User.schema.path('createdAt') as mongoose.SchemaType & {
+          options: { default?: unknown };
+        };
+
+        // Assertion 1: the default is a function
+        expect(typeof createdAtPath.options.default).toBe('function');
+
+        // Assertion 2: calling the default returns a numeric timestamp
+        const result = (createdAtPath.options.default as () => number)();
+        expect(typeof result).toBe('number');
+        expect(Number.isFinite(result)).toBe(true);
+      });
+    });
     it('has trim: true on username path', () => {
       const usernamePath = User.schema.path('username') as mongoose.SchemaType & {
         options: Record<string, unknown>;