test(models): add edge case and validation tests for Notification schema (JhaSourav07#3616)

## Description

Fixes JhaSourav07#2893 

### What Changed
* **Model Validation Tests
(`models/Notification.empty-fallback.test.ts`):** Added a new Vitest
suite specifically targeting backend schema validation, edge cases, and
missing inputs for the Notification model.
* **Required Field Enforcement:** Verified that Mongoose correctly
blocks instantiation when strictly required fields (like `username` and
`email`) are omitted or contain only whitespace (utilizing the schema's
`trim` property).
* **Default Fallback Resilience:** Asserted that unconfigured optional
fields safely default to their intended schema values (e.g., `frequency`
defaulting to `'daily'` and boolean flags defaulting to `true`) when
instantiated with empty configurations.
* **Type & Enum Safety:** Added strict runtime tests to ensure the model
safely catches and rejects invalid enum strings (like `'hourly'`) and
prevents hydration crashes by throwing standard `CastError` exceptions
for invalid data types.
* **Database-Free Testing:** Utilized Mongoose's `.validateSync()`
method to perform lightning-fast, synchronous schema validation without
requiring an active MongoDB connection.

## 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 — Backend model testing only.*

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally (`vitest`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [ ] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have starred the repo.
- [x] I have made sure that I have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

models/Notification.empty-fallback.test.ts

@@ -0,0 +1,86 @@
+import { describe, it, expect } from 'vitest';
+import { Notification } from './Notification';
+
+describe('Notification Model - Edge Cases & Empty/Missing Inputs Verification', () => {
+  // 1. Missing required fields (Empty Object)
+  it('throws validation errors when instantiated with completely empty parameters', () => {
+    const notification = new Notification({});
+
+    // validateSync() allows us to check schema rules without needing a real MongoDB connection
+    const err = notification.validateSync();
+
+    expect(err).toBeDefined();
+    expect(err?.errors.username).toBeDefined();
+    expect(err?.errors.username.message).toContain('Path `username` is required');
+
+    expect(err?.errors.email).toBeDefined();
+    expect(err?.errors.email.message).toContain('Path `email` is required');
+  });
+
+  // 2. Missing optional fields (Verify Default Fallbacks)
+  it('applies default fallback configuration values when optional fields are completely missing', () => {
+    const notification = new Notification({
+      username: 'testuser',
+      email: 'test@example.com',
+    });
+
+    const err = notification.validateSync();
+
+    // Should have no validation errors since required fields are present
+    expect(err).toBeUndefined();
+
+    // Verify all the unconfigured object fields safely fell back to schema defaults
+    expect(notification.frequency).toBe('daily');
+    expect(notification.notifyOnCommit).toBe(true);
+    expect(notification.notifyOnStreak).toBe(true);
+    expect(notification.notifyOnMilestone).toBe(true);
+    expect(notification.isActive).toBe(true);
+    expect(notification.createdAt).toBeDefined();
+    expect(notification.updatedAt).toBeDefined();
+  });
+
+  // 3. Empty string/whitespace handling
+  it('fails validation safely if required strings contain only empty spaces', () => {
+    const notification = new Notification({
+      username: '   ', // Schema has `trim: true`, so this reduces to an empty string
+      email: '   ',
+    });
+
+    const err = notification.validateSync();
+
+    expect(err).toBeDefined();
+    // Mongoose recognizes the trimmed empty string as a missing required field
+    expect(err?.errors.username).toBeDefined();
+    expect(err?.errors.email).toBeDefined();
+  });
+
+  // 4. Invalid Enum Inputs (Missing standard layout state)
+  it('rejects invalid inputs for enumerated fields and prevents database corruption', () => {
+    const notification = new Notification({
+      username: 'testuser',
+      email: 'test@example.com',
+      frequency: 'hourly', // Invalid! Valid options are: realtime, daily, weekly
+    });
+
+    const err = notification.validateSync();
+
+    expect(err).toBeDefined();
+    expect(err?.errors.frequency).toBeDefined();
+    expect(err?.errors.frequency.message).toContain('is not a valid enum value');
+  });
+
+  // 5. Type Casting Failures (Hydration/Runtime Error Resilience)
+  it('gracefully handles invalid types by throwing standard cast errors instead of crashing', () => {
+    const notification = new Notification({
+      username: 'testuser',
+      email: 'test@example.com',
+      notifyOnCommit: 'not-a-boolean-value', // Injecting a raw string into a boolean field
+    });
+
+    const err = notification.validateSync();
+
+    expect(err).toBeDefined();
+    expect(err?.errors.notifyOnCommit).toBeDefined();
+    expect(err?.errors.notifyOnCommit.name).toBe('CastError');
+  });
+});