🚨 Add tests for cache (#1995)

Author: KATO-Hiro

src/test/lib/clients/cache.test.ts

@@ -19,6 +19,7 @@ describe('Cache', () => {
     });
 
     test('expects to throw error if TTL is not positive', () => {
+      expect(() => new Cache<string>(0)).toThrow('TTL must be positive');
       expect(() => new Cache<string>(-1)).toThrow('TTL must be positive');
       expect(() => new Cache<string>(-2)).toThrow('TTL must be positive');
     });
@@ -202,6 +203,12 @@ describe('Cache', () => {
       expect(health.size).toBe(2);
       expect(health.oldestEntry).toBe(new Date('2023-01-01T00:00:00.000Z').getTime());
     });
+
+    test('expects to handle empty cache health information', () => {
+      const cache = new Cache<string>();
+      const health = cache.health;
+      expect(health.size).toBe(0);
+    });
   });
 
   describe('delete and clear', () => {