docs: add parallel test execution documentation and examples

[Wirasm]

Summary

This PR adds comprehensive documentation for parallel test execution using pytest-xdist, demonstrating the project's ability to run tests in parallel to reduce CI/CD execution time and costs. The documentation serves as both a guide for developers and validation that parallel testing infrastructure is properly configured in the Dylan project.

Changes

• Added pytest-xdist as a development dependency
• Created comprehensive TESTING.md documentation with parallel execution section
• Implemented test_parallel_execution.py with validation tests
• Updated CLAUDE.md with parallel execution examples
• Updated pyproject.toml with pytest-xdist dependency
• Created PRP document for parallel execution testing feature

Implementation Details

See: PRPs/features/completed/parallel-execution-test.md

This implementation follows the PRP (Product Requirement Prompt) pattern to ensure:

• Clear documentation of parallel execution capabilities
• Installation and usage examples for pytest-xdist
• Best practices for writing parallel-safe tests
• Performance benefits and cost savings guidance
• Troubleshooting guide for common parallel testing issues

Key Technical Details:

• Parallel execution reduces test execution time by 30-80% for large test suites
• Uses pytest-xdist plugin with -n auto for automatic worker scaling
• All tests are designed for isolation and independence
• Compatible with existing CI/CD pipelines

Testing

• All existing tests pass in sequential mode (uv run pytest)
• All tests pass in parallel mode (uv run pytest -n auto)
• New parallel execution tests added (dylan/tests/test_parallel_execution.py)
• Manual testing completed with performance validation
• Linting and type checking passed

Files Changed

• .ports.env - Port allocation for test services
• CLAUDE.md - Added parallel execution command examples
• PRPs/features/completed/parallel-execution-test.md - Complete PRP document
• TESTING.md - New parallel execution documentation section
• dylan/tests/test_parallel_execution.py - Parallel execution validation tests
• pyproject.toml - Added pytest-xdist dependency

Statistics: 6 files changed, 786 insertions(+), 1 deletion(-)