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_utils.py

@@ -0,0 +1,104 @@
+"""
+工具函数单元测试
+
+测试颜色转换和格式验证函数
+"""
+
+import pytest
+from utils import hex_to_rgb, validate_color
+
+
+class TestHexToRgb:
+    """hex_to_rgb 函数测试类"""
+
+    def test_full_hex_format(self):
+        """测试完整的 #RRGGBB 格式"""
+        assert hex_to_rgb("#ffffff") == (255, 255, 255)
+        assert hex_to_rgb("#000000") == (0, 0, 0)
+        assert hex_to_rgb("#4a90e2") == (74, 144, 226)
+        assert hex_to_rgb("#FF0000") == (255, 0, 0)
+        assert hex_to_rgb("#00FF00") == (0, 255, 0)
+        assert hex_to_rgb("#0000FF") == (0, 0, 255)
+
+    def test_short_hex_format(self):
+        """测试短格式 #RGB"""
+        assert hex_to_rgb("#fff") == (255, 255, 255)
+        assert hex_to_rgb("#000") == (0, 0, 0)
+        assert hex_to_rgb("#abc") == (170, 187, 204)
+        assert hex_to_rgb("#ABC") == (170, 187, 204)
+
+    def test_without_hash_sign(self):
+        """测试没有 # 号"""
+        assert hex_to_rgb("ffffff") == (255, 255, 255)
+        assert hex_to_rgb("4a90e2") == (74, 144, 226)
+
+    def test_invalid_length(self):
+        """测试无效长度"""
+        with pytest.raises(ValueError, match="无效的颜色格式"):
+            hex_to_rgb("#ff")  # 太短
+        with pytest.raises(ValueError, match="无效的颜色格式"):
+            hex_to_rgb("#fffff")  # 5 位
+        with pytest.raises(ValueError, match="无效的颜色格式"):
+            hex_to_rgb("#fffffff")  # 7 位
+
+    def test_invalid_characters(self):
+        """测试无效字符"""
+        with pytest.raises(ValueError):
+            hex_to_rgb("#gggggg")
+        with pytest.raises(ValueError):
+            hex_to_rgb("#xyz")
+
+    def test_mixed_case(self):
+        """测试大小写混合"""
+        assert hex_to_rgb("#AbCdEf") == (171, 205, 239)
+        assert hex_to_rgb("#aBcDeF") == (171, 205, 239)
+
+
+class TestValidateColor:
+    """validate_color 函数测试类"""
+
+    def test_valid_full_hex_colors(self):
+        """测试有效的完整十六进制颜色"""
+        assert validate_color("#ffffff") is True
+        assert validate_color("#000000") is True
+        assert validate_color("#4a90e2") is True
+        assert validate_color("#FF0000") is True
+        assert validate_color("#ABCDEF") is True
+
+    def test_valid_short_hex_colors(self):
+        """测试有效的短格式十六进制颜色"""
+        assert validate_color("#fff") is True
+        assert validate_color("#000") is True
+        assert validate_color("#abc") is True
+        assert validate_color("#ABC") is True
+
+    def test_without_hash_sign(self):
+        """测试没有 # 号"""
+        assert validate_color("ffffff") is False
+        assert validate_color("fff") is False
+
+    def test_invalid_length(self):
+        """测试无效长度"""
+        assert validate_color("#ff") is False
+        assert validate_color("#ffff") is False
+        assert validate_color("#fffff") is False
+        assert validate_color("#fffffff") is False
+
+    def test_invalid_characters(self):
+        """测试无效字符"""
+        assert validate_color("#gggggg") is False
+        assert validate_color("#xyz") is False
+        assert validate_color("red") is False
+        assert validate_color("rgb(255,0,0)") is False
+
+    def test_non_string_input(self):
+        """测试非字符串输入"""
+        assert validate_color(123) is False
+        assert validate_color(None) is False
+        assert validate_color([]) is False
+        assert validate_color({"color": "#fff"}) is False
+
+    def test_empty_string(self):
+        """测试空字符串"""
+        assert validate_color("") is False
+        assert validate_color("#") is False