Support RemoteImage and Base64Image attachments in PrismGateway

[vcprx]

Description

Adds support for RemoteImage and Base64Image attachment types in PrismGateway::toPrismImageAttachments().

Previously, only LocalImage, StoredImage, and UploadedFile were handled. Passing a RemoteImage or Base64Image would fall through to the default branch and throw an InvalidArgumentException, even though the Prism library supports both via PrismImage::fromUrl() and PrismImage::fromBase64().

Changes

• Map RemoteImage → PrismImage::fromUrl($url, $mime)
• Map Base64Image → PrismImage::fromBase64($base64, $mime)

[taylorotwell]

Do providers actually accept this as input?

[vcprx]

Can confirm that Gemini accepts both. Not sure about OpenAI and xAI though...

[vcprx]

No problems with OpenAI as well.