Use atomic writes for config file saves

[thrr87]

Summary

Implement atomic writes for config file operations to prevent corruption from incomplete writes. The changes use a temporary file with os.fsync() to ensure data is written to disk before atomically replacing the target config file with os.replace().

This improves reliability when saving configuration, especially in scenarios with:

• Sudden process termination
• Filesystem errors during write
• Concurrent access to config files

The ensure_config_file() function now delegates to save_config() to use the same atomic write mechanism.

Testing

• python -m pytest

Notes

• Uses tempfile.mkstemp() to create a temporary file in the same directory as the target config file, ensuring they're on the same filesystem for atomic os.replace()
• Includes proper cleanup of temporary files in exception handlers
• The atomic write pattern is now consistent across all config save operations