Add comprehensive tests for PropertyNamingSource with custom json_name

Co-authored-by: Havret <9103861+Havret@users.noreply.github.com>

Author: Copilot

test/Protobuf.System.Text.Json.Tests/PropertyNamingSourceTests.cs

@@ -0,0 +1,109 @@
+using System.Text.Json;
+using System.Text.Json.Protobuf.Tests;
+using Xunit;
+
+namespace Protobuf.System.Text.Json.Tests;
+
+public class PropertyNamingSourceTests
+{
+    [Fact]
+    public void Should_use_custom_json_name_when_PropertyNamingSource_is_ProtobufJsonName()
+    {
+        // Arrange
+        var msg = new MessageWithCustomJsonName
+        {
+            DoubleProperty = 2.5d,
+            StringProperty = "test"
+        };
+        var jsonSerializerOptions = new JsonSerializerOptions();
+        jsonSerializerOptions.AddProtobufSupport(options => options.PropertyNamingSource = PropertyNamingSource.ProtobufJsonName);
+        
+        // Act
+        var serialized = JsonSerializer.Serialize(msg, jsonSerializerOptions);
+
+        // Assert
+        // double_property has json_name = "customDoubleProperty"
+        Assert.Contains("\"customDoubleProperty\"", serialized);
+        // string_property doesn't have json_name, so it should be camelCased to "stringProperty"
+        Assert.Contains("\"stringProperty\"", serialized);
+    }
+
+    [Fact]
+    public void Should_use_proto_field_name_when_PropertyNamingSource_is_ProtobufFieldName()
+    {
+        // Arrange
+        var msg = new MessageWithCustomJsonName
+        {
+            DoubleProperty = 2.5d,
+            StringProperty = "test"
+        };
+        var jsonSerializerOptions = new JsonSerializerOptions();
+        jsonSerializerOptions.AddProtobufSupport(options => options.PropertyNamingSource = PropertyNamingSource.ProtobufFieldName);
+        
+        // Act
+        var serialized = JsonSerializer.Serialize(msg, jsonSerializerOptions);
+
+        // Assert
+        // Both fields should use their original proto field names
+        Assert.Contains("\"double_property\"", serialized);
+        Assert.Contains("\"string_property\"", serialized);
+        // Should NOT use the custom json_name
+        Assert.DoesNotContain("\"customDoubleProperty\"", serialized);
+    }
+
+    [Fact]
+    public void Should_deserialize_using_custom_json_name_when_PropertyNamingSource_is_ProtobufJsonName()
+    {
+        // Arrange
+        var json = "{\"customDoubleProperty\": 2.5, \"stringProperty\": \"test\"}";
+        var jsonSerializerOptions = new JsonSerializerOptions();
+        jsonSerializerOptions.AddProtobufSupport(options => options.PropertyNamingSource = PropertyNamingSource.ProtobufJsonName);
+        
+        // Act
+        var deserialized = JsonSerializer.Deserialize<MessageWithCustomJsonName>(json, jsonSerializerOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.Equal(2.5d, deserialized.DoubleProperty);
+        Assert.Equal("test", deserialized.StringProperty);
+    }
+
+    [Fact]
+    public void Should_deserialize_using_proto_field_name_when_PropertyNamingSource_is_ProtobufFieldName()
+    {
+        // Arrange
+        var json = "{\"double_property\": 2.5, \"string_property\": \"test\"}";
+        var jsonSerializerOptions = new JsonSerializerOptions();
+        jsonSerializerOptions.AddProtobufSupport(options => options.PropertyNamingSource = PropertyNamingSource.ProtobufFieldName);
+        
+        // Act
+        var deserialized = JsonSerializer.Deserialize<MessageWithCustomJsonName>(json, jsonSerializerOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.Equal(2.5d, deserialized.DoubleProperty);
+        Assert.Equal("test", deserialized.StringProperty);
+    }
+
+    [Fact]
+    public void Should_round_trip_with_ProtobufFieldName()
+    {
+        // Arrange
+        var original = new MessageWithCustomJsonName
+        {
+            DoubleProperty = 2.5d,
+            StringProperty = "test"
+        };
+        var jsonSerializerOptions = new JsonSerializerOptions();
+        jsonSerializerOptions.AddProtobufSupport(options => options.PropertyNamingSource = PropertyNamingSource.ProtobufFieldName);
+        
+        // Act
+        var serialized = JsonSerializer.Serialize(original, jsonSerializerOptions);
+        var deserialized = JsonSerializer.Deserialize<MessageWithCustomJsonName>(serialized, jsonSerializerOptions);
+
+        // Assert
+        Assert.NotNull(deserialized);
+        Assert.Equal(original.DoubleProperty, deserialized.DoubleProperty);
+        Assert.Equal(original.StringProperty, deserialized.StringProperty);
+    }
+}

test/Protobuf.System.Text.Json.Tests/Protos/message_with_custom_json_name.proto

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+syntax = "proto3";
+
+option csharp_namespace = "System.Text.Json.Protobuf.Tests";
+
+message MessageWithCustomJsonName {
+  double double_property = 1 [json_name = "customDoubleProperty"];
+
+  string string_property = 2;
+}