Support `multimodalembedding@001` model

Hi team,

I’d like to ask about support for the `multimodalembedding@001` model in the GenAI SDK.

Currently, this model can only be accessed via the Vertex AI SDK. However, according to the documentation, this access method will no longer be available after [June 24, 2026](https://github.com/googleapis/python-aiplatform/blob/7142c625daaabcb0f5a896f046ecd6e0b3958a75/gemini_docs/README.md?plain=1#L263-L271).

At the same time, the model itself is scheduled for retirement much later, on [April 1, 2027](https://docs.cloud.google.com/vertex-ai/generative-ai/docs/learn/model-versions#embeddings-models).

This creates a gap where the model is still active, but there is no clear supported way to access it via the newer SDK.

Right now, calling the model through the GenAI SDK results in a 400 error:

> ClientError: 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': 'No embedding input is provided.', 'status': 'INVALID_ARGUMENT'}}

or if sent the direct HTTP request via the `embedContent` endpoint:

> Publisher Model `projects/.../locations/.../publishers/google/models/multimodalembedding@001` is not supported in the embedContent API.

A similar issue was raised previously: https://github.com/googleapis/python-genai/issues/1975

It was closed based on two points:

1. The model is deprecated
2. The GenAI SDK does not support multimodal embeddings

However, these assumptions no longer seem to apply:

* `multimodalembedding@001` is still active until April 2027
* The GenAI SDK now supports multimodal embeddings (e.g. [gemini-embedding-2-preview](https://ai.google.dev/gemini-api/docs/embeddings#embed-images))

**Would it be possible to support `multimodalembedding@001` in the GenAI SDK, or provide guidance on the recommended migration path during this interim period?**