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Add pytest-based testing framework with 765 test cases to validate Python
code examples and Markdown documentation structure.

Features:
- Python validation: syntax, deprecation warnings, expected output
- Markdown validation: required sections, tables, section order, code refs
- Link validation: internal links, index table, code file references
- Smart CI/CD: tests only changed CWE directories on PRs (~1 min)
- Full suite: runs all tests on pushes to main (~3 min)
- Multi-version: Python 3.9-3.14 testing with tox
- Clean reporting: generate_issue_report.py for actionable summaries
- Auto cleanup: removes test artifacts after test runs

CI/CD Integration:
- GitHub Actions workflow with matrix strategy
- Lychee link checker (internal links in CI, external available locally)
- Coverage reporting to Codecov
- Fast execution with uv package manager

Documentation:
- Comprehensive tests/README.md with usage examples
- Updated CONTRIBUTING.md with testing workflow
- KNOWN_ISSUES.md documents 44 files with pre-existing issues

The framework identifies issues in 44 files (31 documentation files with
missing sections/tables/order issues, 13 Python files with timeouts/warnings)
documented in KNOWN_ISSUES.md. These pre-existing issues can be addressed in
follow-up PRs. All new contributions will be validated automatically.

Closes ossf#1010

Signed-off-by: tommcd <[email protected]>

Change readme reference from 'README.md' to 'tests/README.md' to match
actual file location and fix build error in CI.

Signed-off-by: tommcd <[email protected]>

Mark CWE-390 files with EXPECTED_TIMEOUT to skip timeout tests.
These files intentionally contain infinite loops/blocking operations
for educational purposes as documented in KNOWN_ISSUES.md.

Signed-off-by: tommcd <[email protected]>

Remove invalid 'exist_ok=True' parameter from Path() constructor.
This parameter belongs to Path.mkdir(), not Path(). The invalid
usage triggers DeprecationWarning in Python 3.12+ and will cause
errors in Python 3.14+.

The file is created by open() on the next line, so exist_ok was
never needed here. This is a bug fix with no behavior change.

Signed-off-by: tommcd <[email protected]>

Fix MD012 markdown linting error.

Signed-off-by: tommcd <[email protected]>