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/templates/complex/title-slide-complex.yaml

@@ -0,0 +1,59 @@
+vars:
+  - name: title
+    required: true
+  - name: subtitle
+    required: false
+    default: ""
+  - name: author
+    required: false
+    default: ""
+  - name: date
+    required: false
+    default: ""
+
+elements:
+  # 标题
+  - type: shape
+    box: [0, 0, 10, 5.625]
+    shape: rectangle
+    fill: "#4a90e2"
+
+  # 标题文本
+  - type: text
+    box: [1, 1.5, 8, 1]
+    content: "{title}"
+    font:
+      size: 44
+      bold: true
+      color: "#ffffff"
+      align: center
+
+  # 副标题（条件显示）
+  - type: text
+    box: [1, 2.8, 8, 0.6]
+    content: "{subtitle}"
+    visible: "{subtitle != ''}"
+    font:
+      size: 24
+      color: "#ffffff"
+      align: center
+
+  # 作者
+  - type: text
+    box: [1, 4, 8, 0.5]
+    content: "{author}"
+    visible: "{author != ''}"
+    font:
+      size: 18
+      color: "#e0e0e0"
+      align: center
+
+  # 日期
+  - type: text
+    box: [1, 4.8, 8, 0.4]
+    content: "{date}"
+    visible: "{date != ''}"
+    font:
+      size: 14
+      color: "#cccccc"
+      align: center