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_core/test_exceptions.py

@@ -0,0 +1,37 @@
+"""
+业务异常单元测试
+=============================================================
+
+验证 BizError 及子类的 msg/code 约定：
+业务层 raise BizError → 全局异常处理器转 R.failed。
+"""
+
+from src.core.exceptions import BizError, NotFoundError, PermissionDeniedError
+
+
+class TestBizError:
+    def test_default_values(self):
+        """默认 msg='操作失败'，code=1"""
+        e = BizError()
+        assert e.msg == "操作失败"
+        assert e.code == 1
+
+    def test_custom_msg_and_code(self):
+        e = BizError("自定义错误", code=403)
+        assert e.msg == "自定义错误"
+        assert e.code == 403
+
+    def test_is_exception(self):
+        assert issubclass(BizError, Exception)
+
+
+class TestSubclasses:
+    def test_not_found(self):
+        e = NotFoundError()
+        assert e.msg == "资源不存在"
+        assert e.code == 1
+
+    def test_permission_denied(self):
+        e = PermissionDeniedError()
+        assert e.msg == "权限不足"
+        assert e.code == 403