test(core): add unit tests and test infrastructure

Add test structure:
- Unit tests for BaseEntity
- Unit tests for Guards utility
- Test fixtures with code samples
- Integration and unit test directories

packages/core/tests/unit/domain/BaseEntity.test.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect } from 'vitest';
+import { BaseEntity } from '../../../src/domain/entities/BaseEntity';
+
+class TestEntity extends BaseEntity {
+    constructor(id?: string) {
+        super(id);
+    }
+}
+
+describe('BaseEntity', () => {
+    it('should create an entity with generated id', () => {
+        const entity = new TestEntity();
+        expect(entity.id).toBeDefined();
+        expect(typeof entity.id).toBe('string');
+    });
+
+    it('should create an entity with provided id', () => {
+        const customId = 'custom-id-123';
+        const entity = new TestEntity(customId);
+        expect(entity.id).toBe(customId);
+    });
+
+    it('should have createdAt and updatedAt timestamps', () => {
+        const entity = new TestEntity();
+        expect(entity.createdAt).toBeInstanceOf(Date);
+        expect(entity.updatedAt).toBeInstanceOf(Date);
+    });
+
+    it('should return true when comparing same entity', () => {
+        const entity = new TestEntity();
+        expect(entity.equals(entity)).toBe(true);
+    });
+
+    it('should return true when comparing entities with same id', () => {
+        const id = 'same-id';
+        const entity1 = new TestEntity(id);
+        const entity2 = new TestEntity(id);
+        expect(entity1.equals(entity2)).toBe(true);
+    });
+
+    it('should return false when comparing entities with different ids', () => {
+        const entity1 = new TestEntity();
+        const entity2 = new TestEntity();
+        expect(entity1.equals(entity2)).toBe(false);
+    });
+});