DevUI: Serialize workflow input as string to maintain conformance with OpenAI Responses format (#2021)

Co-authored-by: Victor Dibia <chuvidi2003@gmail.com>

dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/Models/ResponseInput.cs

@@ -182,7 +182,9 @@ internal sealed class ResponseInputJsonConverter : JsonConverter<ResponseInput>
             return messages is not null ? ResponseInput.FromMessages(messages) : null;
         }
 
-        throw new JsonException($"Unexpected token type for ResponseInput: {reader.TokenType}");
+        throw new JsonException(
+            "ResponseInput must be either a string or an array of messages. " +
+            $"Objects are not supported. Received token type: {reader.TokenType}");
     }
 
     /// <inheritdoc/>

dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/OpenAIResponsesSerializationTests.cs

@@ -344,6 +344,20 @@ public sealed class OpenAIResponsesSerializationTests : ConformanceTestBase
         Assert.NotNull(request.Input);
     }
 
+    [Fact]
+    public void Deserialize_InvalidInputObject_ThrowsHelpfulException()
+    {
+        // Arrange
+        const string Json = "{\"model\":\"gpt-4o-mini\",\"input\":{\"input\":\"testing!\"},\"stream\":true}";
+
+        // Act & Assert
+        var exception = Assert.Throws<JsonException>(() =>
+            JsonSerializer.Deserialize(Json, OpenAIHostingJsonContext.Default.CreateResponse));
+
+        Assert.Contains("ResponseInput must be either a string or an array of messages", exception.Message);
+        Assert.Contains("Objects are not supported", exception.Message);
+    }
+
     [Fact]
     public void Deserialize_AllRequests_CanBeDeserialized()
     {