refactor: 测试移至独立子包 simapi_serialization.tests（src/tests/）

- JsonSerializer_test.cj 从 src/json/ 移至 src/tests/，包名 simapi_serialization.tests
- import 简化：测试包直接 import simapi_serialization.*（无循环依赖）
- 测试包不进入库产物（cjpm 自动排除 *_test.cj）
- 16 用例全绿

src/tests/JsonSerializer_test.cj

@@ -8,15 +8,12 @@
  * 运行：cjpm test
  */
 
-package simapi_serialization.json
+package simapi_serialization.tests
 
 import std.collection.*
 import std.unittest.*
 import std.unittest.testmacro.*
-import simapi_serialization.common.SerializerPropertyName
-import simapi_serialization.common.SerializerIgnore
-import simapi_serialization.common.PropertyNamingPolicy
-import simapi_serialization.common.ReflectionCache
+import simapi_serialization.*
 import simapi_serialization.macros.*
 enum Role {
     | Admin