ci: add Python test matrix and expand coverage

[ghostfix-pm]

Summary

• Add pyproject.toml with pytest/ruff config and Python version classifiers
• Expand pyproject.toml with pytest/ruff config and Python version classifiers
• Expand pyproject.toml with pytest/ruff config and Python version classifiers
• Expand .gitignore to cover common Python artifacts (dist/, build/, .eggs/, .mypy_cache/, etc.)
• Add tests/test_eval_helpers_extended.py with 26 new tests covering:
  • format_result edge cases (stderr-only, no elapsedMs, raw errors, line/col)
  • format_process_error variations
  • augmented_path edge cases (empty current, order preservation, tilde expansion)
  • build_eval_argv edge cases (custom timeout, zero timeout, paths with spaces)
  • choose_source edge cases (empty buffer, whitespace-only, multiple with empty)
  • resolve_sema behavior for missing vs real binaries
  • _startupinfo_kwargs Unix behavior
• All 49 tests pass (13 original + 26 new + 10 contract tests)

Test Results

49 passed in 0.06s

Files Changed

File	Δ	What
pyproject.toml	+32	Build config, classifiers, pytest/ruff settings
.gitignore	+16	Expanded Python artifacts
tests/test_eval_helpers_extended.py	+122	26 new test cases
.github/workflows/ci.yml	modified	(removed due to token scope — see note)

Note: CI workflow update was removed because the OAuth token lacks workflow scope. The original validation workflow is preserved in the upstream repo. A follow-up PR can add it with proper permissions.