feat: Implement compact JSON serialization for history entries, improve history loading robustness, and add a new LLM model option.

Serialization/AppJsonSerializerContext.cs

@@ -23,3 +23,9 @@ namespace Toak.Serialization;
 internal partial class AppJsonSerializerContext : JsonSerializerContext
 {
 }
+
+[JsonSourceGenerationOptions(WriteIndented = false, DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull)]
+[JsonSerializable(typeof(Toak.Core.HistoryEntry))]
+internal partial class CompactJsonSerializerContext : JsonSerializerContext
+{
+}