🧪 Add SubDirectoryFixerInstallerService tests

[mleem97]

🎯 What: The testing gap in SubDirectoryFixerInstallerService.EnsureInstalledAsync has been addressed. The service now has a complete set of unit tests covering the standard flows and specifically validating error handling that was previously untested.

📊 Coverage: The test suite SubDirectoryFixerInstallerServiceTests now covers:

• Input validation (Null, empty, whitespace strings)
• Missing GameRoot configuration handling
• Successful installation via payload copy and SHA256 checksum writing
• Safe no-op execution when the plugin is already correctly installed
• Self-updating mechanism when the .sha256 marker hash mismatches the actual payload hash
• 🚨 Missing Error Path: Graceful fallback error reporting (Failed status) when file operations encounter critical issues, such as the UnauthorizedAccessException induced by file locks.

✨ Result: A significant improvement in test coverage. It correctly ensures the installer functions predictably and avoids crashing due to filesystem errors or file locks without resorting to modifying actual machine state.

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PR created automatically by Jules for task 8865762373956984618 started by @mleem97

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