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

pyproject.toml

@@ -8,3 +8,14 @@ dependencies = [
     "flask",
     "watchdog",
 ]
+
+[project.optional-dependencies]
+dev = [
+    "pytest",
+    "pytest-cov",
+    "pytest-mock",
+    "pillow",
+]
+
+[tool.setuptools]
+packages = ["core", "loaders", "validators", "renderers", "preview"]