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/unit/test_loaders/test_yaml_loader.py

@@ -0,0 +1,171 @@
+"""
+YAML 加载器单元测试
+
+测试 load_yaml_file、validate_presentation_yaml 和 validate_template_yaml 函数
+"""
+
+import pytest
+from pathlib import Path
+from loaders.yaml_loader import (
+    load_yaml_file, validate_presentation_yaml,
+    validate_template_yaml, YAMLError
+)
+
+
+class TestLoadYamlFile:
+    """load_yaml_file 函数测试类"""
+
+    def test_load_valid_yaml(self, sample_yaml):
+        """测试加载有效的 YAML 文件"""
+        data = load_yaml_file(sample_yaml)
+        assert isinstance(data, dict)
+        assert "slides" in data
+
+    def test_load_nonexistent_file(self, temp_dir):
+        """测试加载不存在的文件"""
+        nonexistent = temp_dir / "nonexistent.yaml"
+        with pytest.raises(YAMLError, match="文件不存在"):
+            load_yaml_file(nonexistent)
+
+    def test_load_directory_raises_error(self, temp_dir):
+        """测试加载目录会引发错误"""
+        with pytest.raises(YAMLError, match="不是有效的文件"):
+            load_yaml_file(temp_dir)
+
+    def test_load_yaml_with_syntax_error(self, temp_dir):
+        """测试加载包含语法错误的 YAML"""
+        invalid_file = temp_dir / "invalid.yaml"
+        invalid_file.write_text("key: [unclosed list")
+        with pytest.raises(YAMLError, match="YAML 语法错误"):
+            load_yaml_file(invalid_file)
+
+    def test_load_utf8_content(self, temp_dir):
+        """测试加载 UTF-8 编码的内容"""
+        utf8_file = temp_dir / "utf8.yaml"
+        utf8_file.write_text("key: '中文内容'", encoding='utf-8')
+        data = load_yaml_file(utf8_file)
+        assert data["key"] == "中文内容"
+
+
+class TestValidatePresentationYaml:
+    """validate_presentation_yaml 函数测试类"""
+
+    def test_valid_presentation_structure(self):
+        """测试有效的演示文稿结构"""
+        data = {
+            "metadata": {"size": "16:9"},
+            "slides": [{"elements": []}]
+        }
+        # 不应该引发错误
+        validate_presentation_yaml(data)
+
+    def test_missing_slides_field(self):
+        """测试缺少 slides 字段"""
+        data = {"metadata": {"size": "16:9"}}
+        with pytest.raises(YAMLError, match="缺少必需字段 'slides'"):
+            validate_presentation_yaml(data)
+
+    def test_slides_not_a_list(self):
+        """测试 slides 不是列表"""
+        data = {"slides": "not a list"}
+        with pytest.raises(YAMLError, match="'slides' 必须是一个列表"):
+            validate_presentation_yaml(data)
+
+    def test_not_a_dict(self):
+        """测试不是字典"""
+        data = "not a dict"
+        with pytest.raises(YAMLError, match="必须是一个字典对象"):
+            validate_presentation_yaml(data)
+
+    def test_empty_slides_list(self):
+        """测试空的 slides 列表"""
+        data = {"slides": []}
+        # 空列表是有效的
+        validate_presentation_yaml(data)
+
+    def test_with_file_path_in_error(self):
+        """测试错误消息包含文件路径"""
+        data = {"not_slides": []}
+        with pytest.raises(YAMLError) as exc_info:
+            validate_presentation_yaml(data, "test.yaml")
+        assert "test.yaml" in str(exc_info.value)
+
+
+class TestValidateTemplateYaml:
+    """validate_template_yaml 函数测试类"""
+
+    def test_valid_template_structure(self):
+        """测试有效的模板结构"""
+        data = {
+            "vars": [
+                {"name": "title", "required": True}
+            ],
+            "elements": []
+        }
+        # 不应该引发错误
+        validate_template_yaml(data)
+
+    def test_missing_elements_field(self):
+        """测试缺少 elements 字段"""
+        data = {"vars": []}
+        with pytest.raises(YAMLError, match="缺少必需字段 'elements'"):
+            validate_template_yaml(data)
+
+    def test_elements_not_a_list(self):
+        """测试 elements 不是列表"""
+        data = {"elements": "not a list"}
+        with pytest.raises(YAMLError, match="'elements' 必须是一个列表"):
+            validate_template_yaml(data)
+
+    def test_vars_not_a_list(self):
+        """测试 vars 不是列表"""
+        data = {
+            "vars": "not a list",
+            "elements": []
+        }
+        with pytest.raises(YAMLError, match="'vars' 必须是一个列表"):
+            validate_template_yaml(data)
+
+    def test_var_item_not_a_dict(self):
+        """测试变量项不是字典"""
+        data = {
+            "vars": ["not a dict"],
+            "elements": []
+        }
+        with pytest.raises(YAMLError, match="必须是一个字典对象"):
+            validate_template_yaml(data)
+
+    def test_var_item_missing_name(self):
+        """测试变量项缺少 name 字段"""
+        data = {
+            "vars": [{"required": True}],
+            "elements": []
+        }
+        with pytest.raises(YAMLError, match="缺少必需字段 'name'"):
+            validate_template_yaml(data)
+
+    def test_template_without_vars(self):
+        """测试没有 vars 的模板"""
+        data = {"elements": []}
+        # vars 是可选的
+        validate_template_yaml(data)
+
+    def test_not_a_dict(self):
+        """测试不是字典"""
+        data = "not a dict"
+        with pytest.raises(YAMLError, match="必须是一个字典对象"):
+            validate_template_yaml(data)
+
+
+class TestYAMLError:
+    """YAMLError 异常测试类"""
+
+    def test_is_an_exception(self):
+        """测试 YAMLError 是异常类"""
+        error = YAMLError("Test error")
+        assert isinstance(error, Exception)
+
+    def test_can_be_raised_and_caught(self):
+        """测试可以被引发和捕获"""
+        with pytest.raises(YAMLError):
+            raise YAMLError("Test")