Test coverage for core modules

[PyXiion]

Origin of idea

30+ core modules have zero test coverage, including critical orchestrators:

• core/mod_service.py — mod discovery, save pipeline
• core/services/diagnostics_service.py — wires all issue checkers
• core/services/game_launcher.py — launches RimWorld
• core/config.py — app config, VDF parsing, path auto-detection
• core/providers/*.py — mod discovery from game/core/local/steam dirs

Suggestion / proposed solution

Write unit tests with mocked providers and temp fixtures:

Alternatives considered

No response

Additional context

No response