Fix serialization for DALL-E edits

[lzell]

OpenAI expects a multipart upload with part image[] for gpt-image-1 and gpt-image-1-mini. However, they expect part image for dall-e-2. This patch makes that special case adjustment.

Working example to edit a png with dall-e-2 (note that OpenAI requires an alpha channel on your png):

    import AIProxy

    /* Uncomment for BYOK use cases */
    // let openAIService = AIProxy.openAIDirectService(
    //     unprotectedAPIKey: "your-openai-key"
    // )

    /* Uncomment for all other production use cases */
    // let openAIService = AIProxy.openAIService(
    //     partialKey: "partial-key-from-your-developer-dashboard",
    //     serviceURL: "service-url-from-your-developer-dashboard"
    // )

    guard let image = UIImage(named: "my-image") else {
        print("Could not find an image named 'my-image' in your app assets")
        return nil
    }

    guard let pngData = pngDataWithAlpha(image) else {
        print("Could not encode image as png")
        return nil
    }

    let requestBody = OpenAICreateImageEditRequestBody(
        images: [.png(pngData)],
        prompt: "Change the boat mast to red",
        model: .dallE2,
        responseFormat: .b64JSON
    )

    do {
        let response = try await openAIService.createImageEditRequest(
            body: requestBody,
            secondsToWait: 300
        )
        guard let base64Data = response.data.first?.b64JSON,
              let imageData = Data(base64Encoded: base64Data),
              let editedImage = UIImage(data: imageData) else {
            print("Could not create a UIImage out of the base64 returned by OpenAI")
            return nil
        }
        // Do something with 'editedImage'
    } catch AIProxyError.unsuccessfulRequest(let statusCode, let responseBody) {
        print("Received non-200 status code: \(statusCode) with response body: \(responseBody)")
    } catch {
        print("Could not create OpenAI edit image generation: \(error.localizedDescription)")
    }

[lzell]

Closes: #211