Fix ObjectToInferredTypesConverter stack overflow when serializing plain object

[Copilot]

The ObjectToInferredTypesConverter in the JSON serialization documentation was causing a stack overflow when attempting to serialize a plain object instance (e.g., new object()).

Problem

The issue occurred in the Write method:

public override void Write(
    Utf8JsonWriter writer,
    object objectToWrite,
    JsonSerializerOptions options) =>
    JsonSerializer.Serialize(writer, objectToWrite, objectToWrite.GetType(), options);

When objectToWrite is a plain object instance, objectToWrite.GetType() returns typeof(object), which causes the converter to recursively call itself indefinitely, resulting in a stack overflow.

Solution

Added a check for typeof(object) in the Write method. When the object type is exactly object, the method now writes an empty JSON object {} instead of recursively calling JsonSerializer.Serialize:

public override void Write(
    Utf8JsonWriter writer,
    object objectToWrite,
    JsonSerializerOptions options)
{
    var valueType = objectToWrite.GetType();
    if (valueType == typeof(object))
    {
        writer.WriteStartObject();
        writer.WriteEndObject();
        return;
    }

    JsonSerializer.Serialize(writer, objectToWrite, valueType, options);
}

Testing

• ✅ Fixed the stack overflow for new object() serialization (now produces {})
• ✅ Verified original documentation example still works correctly
• ✅ Confirmed type inference functionality is preserved (DateTime, numbers, strings, etc.)
• ✅ Tested edge cases with object properties in complex scenarios

This is a minimal, surgical fix that addresses the specific issue without breaking any existing functionality.

Fixes #43831.

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[BillWagner]

This LGTM @gewarren

Once you and @eiriktsarpalis agree on the style suggestion, go ahead and