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/full_features.yaml

@@ -0,0 +1,71 @@
+metadata:
+  size: 16:9
+
+slides:
+  # 文本元素幻灯片
+  - background:
+      color: "#ffffff"
+    elements:
+      - type: text
+        box: [1, 1, 8, 1]
+        content: "Full Features Test"
+        font:
+          size: 44
+          bold: true
+          color: "#333333"
+          align: center
+
+      - type: text
+        box: [1, 2.5, 8, 0.5]
+        content: "Testing all element types"
+        font:
+          size: 18
+          italic: true
+          color: "#666666"
+          align: center
+
+  # 图片元素幻灯片
+  - elements:
+      - type: image
+        box: [1, 1, 4, 3]
+        src: "test_image.png"
+
+  # 形状元素幻灯片
+  - elements:
+      - type: shape
+        box: [1, 1, 2, 1]
+        shape: rectangle
+        fill: "#4a90e2"
+        line:
+          color: "#000000"
+          width: 2
+
+      - type: shape
+        box: [4, 1, 2, 1]
+        shape: ellipse
+        fill: "#e24a4a"
+        line:
+          color: "#000000"
+          width: 1
+
+      - type: shape
+        box: [7, 1, 2, 1]
+        shape: rounded_rectangle
+        fill: "#4ae290"
+        line:
+          color: "#000000"
+          width: 2
+
+  # 表格元素幻灯片
+  - elements:
+      - type: table
+        position: [1, 1]
+        col_widths: [2, 2, 2]
+        data:
+          - ["Header 1", "Header 2", "Header 3"]
+          - ["Data 1", "Data 2", "Data 3"]
+          - ["Data 4", "Data 5", "Data 6"]
+        style:
+          font_size: 14
+          header_bg: "#4a90e2"
+          header_color: "#ffffff"