docs+tests: P8 测试与文档（CHANGELOG/CONTRIBUTING + 补充单测）

- CHANGELOG.md / CONTRIBUTING.md
- 新增 exceptions / case_converter 单元测试
- pytest 12 → 22 passed；全项目 flake8 0

Co-Authored-By: Claude <noreply@anthropic.com>

tests/unit/test_common/test_case_converter.py

@@ -0,0 +1,38 @@
+"""
+命名转换工具单元测试
+=============================================================
+
+- convert_camel_case / convert_snake_case：单字符串转换
+- convert_camel_to_snake / convert_snake_to_camel：字典键递归转换
+"""
+
+from src.common.utils.case_converter import (
+    convert_camel_case,
+    convert_snake_case,
+    convert_camel_to_snake,
+    convert_snake_to_camel,
+)
+
+
+class TestSingleString:
+    def test_snake_to_camel(self):
+        assert convert_camel_case("user_id") == "userId"
+
+    def test_camel_to_snake(self):
+        assert convert_snake_case("userId") == "user_id"
+
+
+class TestDictKeys:
+    def test_camel_to_snake_dict(self):
+        data = {"userId": 1, "userInfo": {"realName": "张三"}}
+        assert convert_camel_to_snake(data) == {"user_id": 1, "user_info": {"real_name": "张三"}}
+
+    def test_snake_to_camel_dict(self):
+        data = {"user_id": 1, "user_info": {"real_name": "张三"}}
+        assert convert_snake_to_camel(data) == {"userId": 1, "userInfo": {"realName": "张三"}}
+
+    def test_roundtrip(self):
+        source = {"user_id": 1, "role_ids": [1, 2]}
+        camel = convert_snake_to_camel(source)
+        assert camel == {"userId": 1, "roleIds": [1, 2]}
+        assert convert_camel_to_snake(camel) == source