Include final-bar open trades in results

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Summary

• process orders placed during the final strategy iteration
• include still-open trades in results when finalize_trades=False
• represent their exit fields with NaN and keep exposure and indicator columns valid
• add regression coverage for final-bar orders and update the affected trade-count expectation

Fixes #1342.

Testing

• python -m backtesting.test (81 tests: 80 passed, 1 platform-specific skip)
• flake8 backtesting/_stats.py backtesting/backtesting.py backtesting/test/_test.py

Local limitation

mypy backtesting could not start because Windows Application Control blocked mypy's native DLL. GitHub Actions should provide the authoritative type-check result.

AI assistance

OpenAI Codex assisted with issue discovery, implementation, tests, and validation. I reviewed the change and test results.