Fix streaming function call decoding error when 'name' field is missing

Make 'name' optional in FunctionCallArgumentsDoneEvent to handle cases
where OpenAI API omits this field in response.function_call_arguments.done
streaming events.

Fixes #188

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jamesrochabrun

Sources/OpenAI/Public/ResponseModels/Response/ResponseStreamEvent.swift

@@ -607,7 +607,7 @@ public struct FunctionCallArgumentsDoneEvent: Decodable {
   public let type: String
   public let itemId: String
   public let outputIndex: Int
-  public let name: String
+  public let name: String?
   public let arguments: String
   public let sequenceNumber: Int?