Add multimodal embeddings support

[virgildotcodes]

Closes #308

Summary

Laravel AI currently only supports text inputs for embeddings. Prism added multimodal embeddings support, but the Laravel SDK did not expose it cleanly and had a few provider-specific gaps.

This PR adds multimodal embeddings input support for embeddings, including:

• images
• audio
• documents
• video

It also validates unsupported provider / model combinations early, preserves the original media source when mapping Prism embeddings inputs, and avoids fetching remote media when generating embeddings cache keys.

Examples

Gemini multimodal embeddings

use Laravel\Ai\Embeddings;
use Laravel\Ai\Files\Image;

$response = Embeddings::for([
    Image::fromPath('/path/to/image.png'),
])->generate(provider: 'gemini', model: 'gemini-embedding-2-preview');

Voyage AI image embeddings

use Laravel\Ai\Embeddings;
use Laravel\Ai\Files\Image;

$response = Embeddings::for([
    Image::fromUrl('https://example.com/image.png'),
])->dimensions(1024)->generate(
    provider: 'voyageai',
    model: 'voyage-multimodal-3',
);

Changes

• widen embeddings inputs to accept text, images, audio, documents, and video
• add explicit validation for unsupported provider / input combinations
• resolve Gemini provider-backed files to file URIs before sending them to Prism
• preserve remote, local, stored, and base64 media sources when converting embeddings inputs
• avoid remote fetches when building cache keys for remote embeddings inputs

Notes

• Gemini multimodal embeddings require gemini-embedding-2-preview
• Voyage AI currently supports text and image embeddings inputs