[API nodes] add support for "@image" reference format in Kling Omni API nodes

[bigcat88]

This PR adds support for the Kling app–style prompt syntax (e.g. @image, @image2, @video) to the Kling Omni Pro nodes in ComfyUI. Users can now reference images and videos in prompts the same way they do in the native Kling app, while the nodes still talk to the Kling API using the existing <<<image_1>>> / <<<video_1>>> format.

It also scaffolds a new Omni Image node, which is currently commented out until backend support is available.
Once the backend is ready, we can uncomment the registration & test it - and the node should be ready to use.

API Node PR Checklist

Scope

• Is API Node Change

Pricing & Billing

• Need pricing update
• No pricing update

If Need pricing update:

• Metronome rate cards updated
• Auto‑billing tests updated and passing

QA

• QA done
• QA not required

Comms

• Informed Kosinkadink