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/shared/Guards.test.ts

@@ -0,0 +1,57 @@
+import { describe, it, expect } from 'vitest';
+import { Guards } from '../../../src/shared/utils/Guards';
+
+describe('Guards', () => {
+    describe('isNullOrUndefined', () => {
+        it('should return true for null', () => {
+            expect(Guards.isNullOrUndefined(null)).toBe(true);
+        });
+
+        it('should return true for undefined', () => {
+            expect(Guards.isNullOrUndefined(undefined)).toBe(true);
+        });
+
+        it('should return false for other values', () => {
+            expect(Guards.isNullOrUndefined(0)).toBe(false);
+            expect(Guards.isNullOrUndefined('')).toBe(false);
+            expect(Guards.isNullOrUndefined(false)).toBe(false);
+        });
+    });
+
+    describe('isString', () => {
+        it('should return true for strings', () => {
+            expect(Guards.isString('hello')).toBe(true);
+            expect(Guards.isString('')).toBe(true);
+        });
+
+        it('should return false for non-strings', () => {
+            expect(Guards.isString(123)).toBe(false);
+            expect(Guards.isString(null)).toBe(false);
+        });
+    });
+
+    describe('isEmpty', () => {
+        it('should return true for empty strings', () => {
+            expect(Guards.isEmpty('')).toBe(true);
+        });
+
+        it('should return true for empty arrays', () => {
+            expect(Guards.isEmpty([])).toBe(true);
+        });
+
+        it('should return true for empty objects', () => {
+            expect(Guards.isEmpty({})).toBe(true);
+        });
+
+        it('should return true for null/undefined', () => {
+            expect(Guards.isEmpty(null)).toBe(true);
+            expect(Guards.isEmpty(undefined)).toBe(true);
+        });
+
+        it('should return false for non-empty values', () => {
+            expect(Guards.isEmpty('text')).toBe(false);
+            expect(Guards.isEmpty([1])).toBe(false);
+            expect(Guards.isEmpty({ key: 'value' })).toBe(false);
+        });
+    });
+});