Extract static initialization logic from instance method OnEngineInit

[Copilot]

Addresses code smell: instance method OnEngineInit() was directly writing to static fields (_initialized, Config, _patchesApplied, _shutdownHookRegistered).

Changes:

• Extracted initialization logic to new static method InitializeStatic(ModConfiguration? config)
• OnEngineInit() now delegates immediately: InitializeStatic(GetConfiguration())
• Improved null handling: validate config parameter before assignment instead of using null-forgiving operator followed by null check

Before:

public override void OnEngineInit()
{
    if (_initialized) return;
    Config = GetConfiguration()!;
    // ... writes to static fields throughout
}

After:

public override void OnEngineInit()
{
    InitializeStatic(GetConfiguration());
}

private static void InitializeStatic(ModConfiguration? config)
{
    if (_initialized) return;
    if (config == null) { Error("..."); return; }
    Config = config;
    // ... static field writes now in static context
}

Thread safety and initialization logic unchanged.

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