Handle LoadOrCreate deserialization of JSON null via existing recovery path

[Copilot]

AppData<T>.LoadOrCreate(...) could throw NullReferenceException when the settings file contained literal JSON null because Deserialize<T> returned null without throwing. This change treats that case as recoverable corruption so the existing backup/new-instance flow is used consistently.

• Deserialization null handling

  • Updated LoadOrCreate to convert a null deserialization result into JsonException, keeping control flow inside the existing corruption recovery logic.

• Regression coverage

  • Added a focused test for a settings file containing exactly null, asserting LoadOrCreate() recovers to a valid default instance rather than crashing.

newAppData = JsonSerializer.Deserialize<T>(jsonString, AppData.JsonSerializerOptions)
	?? throw new JsonException("Deserialized settings file to null.");

• Fixes LoadOrCreate throws NullReferenceException instead of recovering when the settings file contains JSON null #295

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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