feat(voyageai): add output_dimension support for embeddings

[markritterman]

Description

Adds outputDimension provider option support to the VoyageAI embeddings handler, allowing control over the number of dimensions in embedding responses.

Voyage AI models like voyage-4 and voyage-multimodal-3.5 support Matryoshka-style dimension truncation via the output_dimension API parameter. This was previously not forwarded by the Prism handler.

Usage

Prism::embeddings()
    ->using(Provider::VoyageAI, 'voyage-4')
    ->fromInput('some text')
    ->withProviderOptions(['outputDimension' => 2048])
    ->asEmbeddings();

When outputDimension is not set, the parameter is omitted from the request (no change in default behaviour).

Tests

• Verifies output_dimension is included in the API request when outputDimension provider option is set
• Verifies output_dimension is omitted from the API request when not set
• All existing VoyageAI embeddings tests continue to pass