feat(utils): add vision model type

[max-wittig]

FILL IN THE PR DESCRIPTION HERE

FIX #xxxx (link existing issues this PR will resolve)

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[gemini-code-assist]

Summary of Changes

Hello @max-wittig, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

I've implemented changes to the vllm_router to introduce explicit support for vision models. This involves defining a new ModelType for vision and providing a concrete example of a multimodal request payload, which will allow the router to properly handle and test requests for models that can process both text and image inputs.

Highlights

• New Model Type Definition: I've introduced a new vision member to the ModelType enum in src/vllm_router/utils.py. This new type is associated with the /v1/chat/completions endpoint, indicating its use for chat-based vision models.
• Multimodal Test Payload: I've extended the get_test_payload static method to include a sample request body for the newly added ModelType.vision. This payload demonstrates a multimodal input structure, including both text and a base64-encoded image URL, which is crucial for testing vision model capabilities.

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[gemini-code-assist]

Code Review

This pull request introduces a new vision model type. However, due to a shared value with the existing chat model type in the ModelType enum, the new type is created as an alias, which breaks the intended functionality. I've provided a critical feedback with a suggestion to resolve this by making the enum value unique.

[max-wittig]

@gemini-code-assist review again

[gemini-code-assist]

Code Review

This pull request introduces a vision model type and refactors the ModelType enum to better separate concerns. The changes are generally good, but there are a few opportunities for improvement in code conciseness and idiomatic Python usage. I've suggested combining match cases and correcting how static methods are called.