fix: 修复 7 个失败测试和 1 个错误测试

- 修复 conftest_pptx.py 中元素类型检测：使用 has_text_frame 替代不存在的 MSO_SHAPE.TEXT_BOX
- 修复 test_presentation.py 中 3 个测试：使用对象属性访问替代字典访问
- 修复 unit/test_presentation.py 中路径比较问题
- 添加缺失的 mock_template_class fixture

测试通过率: 316/316 (100%)
代码覆盖率: 94%

tests/integration/test_presentation.py

@@ -87,9 +87,7 @@ slides:
         # 模板变量应该被替换
         elements = rendered["elements"]
         title_elem = next(
-            e
-            for e in elements
-            if e.get("type") == "text" and "Test Title" in e.get("content", "")
+            e for e in elements if e.type == "text" and "Test Title" in e.content
         )
         assert title_elem is not None
 
@@ -157,8 +155,8 @@ slides:
 
         # 检查变量是否被正确替换
         elements = rendered["elements"]
-        assert any("My Title" in e.get("content", "") for e in elements)
-        assert any("My Subtitle" in e.get("content", "") for e in elements)
+        assert any("My Title" in e.content for e in elements)
+        assert any("My Subtitle" in e.content for e in elements)
 
 
 class TestPresentationWithoutTemplate:
@@ -187,4 +185,4 @@ slides:
 
         # 元素应该直接被渲染
         assert len(rendered["elements"]) == 1
-        assert rendered["elements"][0]["content"] == "Direct Text"
+        assert rendered["elements"][0].content == "Direct Text"