fix(logging): close spdlog file handles before drop

[cloudQuant]

Summary

• Close and unregister each spdlog logger before temporary-directory cleanup, rollover, or handler shutdown so Windows can release the file handle.
• Probe spdlog through the same runtime availability check used by the backend selector, and skip symlink-only assertions when Windows policy denies symlink creation.
• This change only manages log files; it adds no provider calls, broker writes, order/cancel dispatch, or private configuration.

Verification

• python -m pytest -p no:asyncio tests/unit/utils/test_logging_split_files.py -q --tb=short — 53 passed, 4 skipped.
• ruff check backtrader/utils/log_message.py tests/unit/utils/test_logging_split_files.py — passed.
• git diff --check — passed.