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_validators/test_validator.py

@@ -0,0 +1,119 @@
+"""
+主验证器单元测试
+
+测试 Validator 类的协调和分类功能
+"""
+
+import pytest
+from pathlib import Path
+from validators.validator import Validator
+from validators.result import ValidationResult
+
+
+class TestValidator:
+    """Validator 测试类"""
+
+    def test_init(self):
+        """测试初始化"""
+        validator = Validator()
+        assert validator.SLIDE_SIZES == {
+            "16:9": (10, 5.625),
+            "4:3": (10, 7.5)
+        }
+
+    def test_validate_valid_yaml(self, sample_yaml):
+        """测试验证有效的 YAML 文件"""
+        validator = Validator()
+        result = validator.validate(sample_yaml)
+        assert isinstance(result, ValidationResult)
+        assert result.valid is True
+
+    def test_validate_nonexistent_file(self, temp_dir):
+        """测试验证不存在的文件"""
+        validator = Validator()
+        nonexistent = temp_dir / "nonexistent.yaml"
+        result = validator.validate(nonexistent)
+        assert result.valid is False
+        assert len(result.errors) > 0
+
+    def test_categorize_issues_error(self):
+        """测试分类问题为错误"""
+        validator = Validator()
+        from validators.result import ValidationIssue
+
+        errors = []
+        warnings = []
+        infos = []
+
+        issue = ValidationIssue("ERROR", "Test", "", "ERR")
+        validator._categorize_issues([issue], errors, warnings, infos)
+
+        assert len(errors) == 1
+        assert len(warnings) == 0
+        assert len(infos) == 0
+
+    def test_categorize_issues_warning(self):
+        """测试分类问题为警告"""
+        validator = Validator()
+        from validators.result import ValidationIssue
+
+        errors = []
+        warnings = []
+        infos = []
+
+        issue = ValidationIssue("WARNING", "Test", "", "WARN")
+        validator._categorize_issues([issue], errors, warnings, infos)
+
+        assert len(errors) == 0
+        assert len(warnings) == 1
+        assert len(infos) == 0
+
+    def test_categorize_issues_info(self):
+        """测试分类问题为提示"""
+        validator = Validator()
+        from validators.result import ValidationIssue
+
+        errors = []
+        warnings = []
+        infos = []
+
+        issue = ValidationIssue("INFO", "Test", "", "INFO")
+        validator._categorize_issues([issue], errors, warnings, infos)
+
+        assert len(errors) == 0
+        assert len(warnings) == 0
+        assert len(infos) == 1
+
+    def test_categorize_mixed_issues(self):
+        """测试分类混合问题"""
+        validator = Validator()
+        from validators.result import ValidationIssue
+
+        errors = []
+        warnings = []
+        infos = []
+
+        issues = [
+            ValidationIssue("ERROR", "Error", "", "ERR"),
+            ValidationIssue("WARNING", "Warning", "", "WARN"),
+            ValidationIssue("INFO", "Info", "", "INFO"),
+        ]
+        validator._categorize_issues(issues, errors, warnings, infos)
+
+        assert len(errors) == 1
+        assert len(warnings) == 1
+        assert len(infos) == 1
+
+
+class TestSlideSizes:
+    """幻灯片尺寸常量测试"""
+
+    def test_16_9_size(self):
+        """测试 16:9 尺寸"""
+        validator = Validator()
+        assert validator.SLIDE_SIZES["16:9"] == (10, 5.625)
+
+    def test_4_3_size(self):
+        """测试 4:3 尺寸"""
+        validator = Validator()
+        assert validator.SLIDE_SIZES["4:3"] == (10, 7.5)