fix(cache): exists 排除空值标记,与 get 口径一致

exists 原用 hasKey,会把持有空值标记的 key 判为"存在"(true),而 get 对同一 key
返回 null,二者口径打架。改为读值判断:空标记/缺失均返回 false,真实值返回 true。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/test/java/com/njcn/middle/cache/template/RedisCacheEnhanceTemplateTest.java

@@ -23,6 +23,7 @@ import java.util.function.Consumer;
 import java.util.function.Supplier;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTimeoutPreemptively;
@@ -318,4 +319,24 @@ class RedisCacheEnhanceTemplateTest {
     void constructorAcceptsDefaults() {
         new RedisCacheEnhanceTemplate(redis, keyBuilder, codec, new RedisCacheProperties());
     }
+
+    // ---- #7: exists 排除空值标记(与 get 口径一致) ----
+
+    @Test
+    void existsTrueForRealValue() {
+        when(valueOps.get("cache:user:1")).thenReturn(JSON.toJSONString(new User(1L, "a")));
+        assertTrue(template.exists("user:1"));
+    }
+
+    @Test
+    void existsFalseForNullSentinel() {
+        when(valueOps.get("cache:user:1")).thenReturn(CacheConstant.NULL_SENTINEL);
+        assertFalse(template.exists("user:1"));
+    }
+
+    @Test
+    void existsFalseForMissing() {
+        when(valueOps.get("cache:user:1")).thenReturn(null);
+        assertFalse(template.exists("user:1"));
+    }
 }