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/test_template.yaml

@@ -0,0 +1,23 @@
+vars:
+  - name: title
+    required: true
+  - name: subtitle
+    required: false
+    default: ""
+
+elements:
+  - type: text
+    box: [1, 2, 8, 1]
+    content: "{title}"
+    font:
+      size: 44
+      bold: true
+      align: center
+
+  - type: text
+    box: [1, 3.5, 8, 0.5]
+    content: "{subtitle}"
+    visible: "{subtitle != ''}"
+    font:
+      size: 24
+      align: center