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/yaml_samples/edge_cases/all_element_types_one_slide.yaml

@@ -0,0 +1,29 @@
+metadata:
+  size: 16:9
+
+slides:
+  - elements:
+      - type: text
+        box: [1, 1, 3, 0.5]
+        content: "All Element Types One Slide"
+        font:
+          size: 18
+          color: "#333333"
+
+      - type: image
+        box: [1, 2, 2, 2]
+        src: "test_image.png"
+
+      - type: shape
+        box: [4, 2, 2, 1]
+        shape: rectangle
+        fill: "#4a90e2"
+
+      - type: table
+        position: [1, 4.5]
+        col_widths: [2, 2, 2]
+        data:
+          - ["H1", "H2", "H3"]
+          - ["D1", "D2", "D3"]
+        style:
+          font_size: 12

tests/fixtures/yaml_samples/edge_cases/empty_content.yaml

@@ -0,0 +1,10 @@
+metadata:
+  size: 16:9
+
+slides:
+  - elements:
+      - type: text
+        box: [1, 1, 8, 1]
+        content: ""
+        font:
+          size: 18

tests/fixtures/yaml_samples/edge_cases/empty_font_attributes.yaml

@@ -0,0 +1,15 @@
+metadata:
+  size: 16:9
+
+slides:
+  - elements:
+      # 空字体属性
+      - type: text
+        box: [1, 1, 3, 0.5]
+        content: "Empty Font Attributes"
+        font: {}
+
+      # 空字体
+      - type: text
+        box: [1, 2, 3, 0.5]
+        content: "No Font Field"

tests/fixtures/yaml_samples/edge_cases/mixed_color_formats.yaml

@@ -0,0 +1,20 @@
+metadata:
+  size: 16:9
+
+slides:
+  - elements:
+      # 混合颜色格式
+      - type: text
+        box: [1, 1, 3, 0.5]
+        content: "Mixed Color Formats"
+        font:
+          size: 18
+          color: "#fff"
+
+      - type: shape
+        box: [1, 2, 2, 1]
+        shape: rectangle
+        fill: "#000000"
+        line:
+          color: "#abc"
+          width: 2

tests/fixtures/yaml_samples/edge_cases/negative_position.yaml

@@ -0,0 +1,11 @@
+metadata:
+  size: 16:9
+
+slides:
+  - elements:
+      - type: text
+        box: [-1, -1, 2, 1]
+        content: "Negative Position"
+        font:
+          size: 18
+          color: "#333333"

tests/fixtures/yaml_samples/edge_cases/special_chars.yaml

@@ -0,0 +1,10 @@
+metadata:
+  size: 16:9
+
+slides:
+  - elements:
+      - type: text
+        box: [1, 1, 8, 2]
+        content: "Special chars: < > & \" ' \\n \\t @#$%^&*()"
+        font:
+          size: 18

tests/fixtures/yaml_samples/edge_cases/unicode.yaml

@@ -0,0 +1,10 @@
+metadata:
+  size: 16:9
+
+slides:
+  - elements:
+      - type: text
+        box: [1, 1, 8, 1]
+        content: "Unicode 测试: 你好世界 🌍"
+        font:
+          size: 24

tests/fixtures/yaml_samples/edge_cases/very_large_font.yaml

@@ -0,0 +1,11 @@
+metadata:
+  size: 16:9
+
+slides:
+  - elements:
+      - type: text
+        box: [0, 0, 1, 1]
+        content: "Very Large Font"
+        font:
+          size: 200
+          color: "#333333"

tests/fixtures/yaml_samples/edge_cases/zero_size_box.yaml

@@ -0,0 +1,10 @@
+metadata:
+  size: 16:9
+
+slides:
+  - elements:
+      - type: text
+        box: [1, 1, 0, 0]
+        content: "Zero Size Box"
+        font:
+          size: 18