Update src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs

Co-authored-by: Copilot <[email protected]>

Author: stephentoub

src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs

@@ -877,7 +877,7 @@ static FunctionCallOutputResponseItem SerializeAIContent(string callId, IEnumera
                                             {
                                                 Type = "input_file",
                                                 FileData = dc.Uri, // contrary to the docs, file_data is expected to be a data URI, not just the base64 portion
-                                                FileName = dc.Name ?? Guid.NewGuid().ToString("N"), // contrary to the docs, file_name is required
+                                                FileName = dc.Name ?? $"file_{Guid.NewGuid():N}", // contrary to the docs, file_name is required
                                             });
                                             break;