Add PropertyNamingSource enum to support proto field names

[Copilot]

Extends JSON property naming control by introducing a PropertyNamingSource enum that replaces the boolean UseProtobufJsonNames flag, adding support for using original .proto field names.

Changes

• New PropertyNamingSource enum with three options:

  • Default - Uses PropertyNamingPolicy (existing default)
  • ProtobufJsonName - Uses protobuf JsonName (camelCase or custom json_name)
  • ProtobufFieldName - Uses original .proto field name (e.g., double_property)

• Backward compatibility: UseProtobufJsonNames marked obsolete, maps to new enum

• Updated ProtobufConverter: Refactored GetConvertNameFunc to use switch on enum, accessing descriptor.Name for field names

Usage

var options = new JsonSerializerOptions();
options.AddProtobufSupport(opts => 
{
    opts.PropertyNamingSource = PropertyNamingSource.ProtobufFieldName;
});

// For a field defined as "double_property" in .proto:
var json = JsonSerializer.Serialize(msg, options);
// Result: {"double_property": 2.5} instead of {"doubleProperty": 2.5}

Works correctly with custom json_name in .proto files - ProtobufFieldName uses the original field name regardless of json_name overrides.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Use names defined in .proto files as JSON property names to override PropertyNamingPolicy</issue_title>
<issue_description>## Feature description

Currently, the UseProtobufJsonNames flag allows overriding the PropertyNamingPolicy from System.Text.Json by using the protobuf JsonName.

We’d like to extend this functionality to support using the original field names defined directly in .proto files instead of the computed JsonName values.</issue_description>

<agent_instructions>try to implement this. add new enum that will obsolete the existing flag </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Use names defined in .proto files as JSON property names to override PropertyNamingPolicy #69

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[Havret]

LGTM ❤️