System.Text.Json source-gen docs: prefer JsonTypeInfo overloads and remove options/context overload examples

[Copilot]

The source-generation article still demonstrated JsonSerializerOptions and JsonSerializerContext overloads as primary usage, which can produce trimming warnings and misleading Native AOT guidance. This update shifts examples and guidance to JsonTypeInfo<T>-first usage and documents the options-based workaround pattern when options flow is required.

• Default guidance now centers on JsonTypeInfo<T>

  • Updated the “Use source-generation defaults” and serialization/deserialization example sections to remove context/options-overload emphasis.
  • Replaced option-overload examples with patterns that resolve JsonTypeInfo<T> first, then call JsonSerializer type-info overloads.

• Removed obsolete/non-preferred overload examples from snippets

  • BothModesNoOptions.cs: removed SerializeWithContext / DeserializeWithContext and direct options-overload samples.
  • Added SerializeWithTypeInfoFromOptions / DeserializeWithTypeInfoFromOptions snippets using options.GetTypeInfo(...).
  • SerializeOnlyWithOptions.cs: removed context-overload sample block.
  • RoundtripEnumSourceGeneration.cs: replaced options-overload enum serialization calls with generated JsonTypeInfo<T> calls.

• Added Native AOT footnote for options-overload behavior

  • Documented why options-only overloads are unsuitable in source-gen Native AOT scenarios (historical reflection fallback for backward compatibility).
  • Added explicit workaround guidance for options-driven flows.

JsonTypeInfo<MyType> typeInfo =
    (JsonTypeInfo<MyType>)options.GetTypeInfo(typeof(MyType));

string json = JsonSerializer.Serialize(value, typeInfo);
MyType? model = JsonSerializer.Deserialize(json, typeInfo);