Migrate from Newtonsoft.Json to System.Text.Json

[jonathan-kosgei]

Replace Newtonsoft.Json 12.0.2 (which has a known high severity vulnerability) with System.Text.Json 4.7.2. This removes the external dependency for consumers on modern .NET while resolving GHSA-5crp-9r3c-p9vr.

• Replace [JsonProperty] with [JsonPropertyName] across all models
• Rewrite internal JsonSerializer to use System.Text.Json APIs
• Update IPLookupResult.FieldName reflection to use JsonPropertyNameAttribute
• Handle null-ignoring via JsonSerializerOptions.IgnoreNullValues
• Update csproj and nuspec dependencies

Closes #12
Closes #2

https://claude.ai/code/session_013C9LnDefoZu8s7V61xAfGR

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