test: add comprehensive pytest test suite

Add complete test infrastructure for yaml2pptx project with 245+ tests
covering unit, integration, and end-to-end scenarios.

Test structure:
- Unit tests: elements, template system, validators, loaders, utils
- Integration tests: presentation and rendering flows
- E2E tests: CLI commands (convert, check, preview)

Key features:
- PptxFileValidator for Level 2 PPTX validation (file structure,
  element count, content matching, position tolerance)
- Comprehensive fixtures for test data consistency
- Mock-based testing for external dependencies
- Test images generated with PIL/Pillow
- Boundary case coverage for edge scenarios

Dependencies added:
- pytest, pytest-cov, pytest-mock
- pillow (for test image generation)

Documentation updated:
- README.md: test running instructions
- README_DEV.md: test development guide

Co-authored-by: OpenSpec change: add-comprehensive-tests

README.md

@@ -354,6 +354,44 @@ yaml2pptx 采用模块化架构，易于扩展：
 
 依赖在 pyproject.toml 中声明，由 uv 自动管理，无需手动安装。
 
+## 🧪 测试
+
+项目包含完整的测试套件，使用 pytest 框架。
+
+### 运行测试
+
+```bash
+# 安装测试依赖
+uv pip install -e ".[dev]"
+
+# 运行所有测试
+uv run pytest
+
+# 运行特定类型的测试
+uv run pytest tests/unit/       # 单元测试
+uv run pytest tests/integration/  # 集成测试
+uv run pytest tests/e2e/        # 端到端测试
+
+# 运行特定测试文件
+uv run pytest tests/unit/test_elements.py
+
+# 显示详细输出
+uv run pytest -v
+
+# 显示测试覆盖率
+uv run pytest --cov=. --cov-report=html
+```
+
+### 测试结构
+
+```
+tests/
+├── unit/           # 单元测试 - 测试各模块独立功能
+├── integration/    # 集成测试 - 测试模块间协作
+├── e2e/            # 端到端测试 - 测试完整用户场景
+└── fixtures/       # 测试数据
+```
+
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