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

tests/fixtures/create_test_images.py

@@ -0,0 +1,34 @@
+"""
+创建测试图片的辅助脚本
+"""
+
+from PIL import Image, ImageDraw
+from pathlib import Path
+
+# 确保目录存在
+images_dir = Path(__file__).parent.parent / "fixtures" / "images"
+images_dir.mkdir(exist_ok=True)
+
+# 创建一个简单的红色图片
+img = Image.new('RGB', (100, 100), color='red')
+img.save(images_dir / "test_image.png")
+
+# 创建一个较大的图片
+large_img = Image.new('RGB', (800, 600), color='blue')
+large_img.save(images_dir / "large_image.png")
+
+# 创建一个带有透明度的图片（PNG）
+transparent_img = Image.new('RGBA', (100, 100), (255, 0, 0, 128))
+transparent_img.save(images_dir / "transparent_image.png")
+
+# 创建一个小图片
+small_img = Image.new('RGB', (50, 50), color='green')
+small_img.save(images_dir / "small_image.png")
+
+# 创建一个带文字的图片
+text_img = Image.new('RGB', (200, 100), color='white')
+draw = ImageDraw.Draw(text_img)
+draw.text((10, 30), "Test Image", fill='black')
+text_img.save(images_dir / "text_image.png")
+
+print(f"Created test images in {images_dir}")