fix(vllm-metal): enable tool calling support in backend args

[doringeman]

Fixes

$ ldm run huggingface.co/mlx-community/SmolLM2-135M-Instruct hi
Failed to generate a response: error response: status=400 body={"error":{"message":"\"auto\" tool choice requires --enable-auto-tool-choice and --tool-call-parser to be set","type":"BadRequestError","param":null,"code":400}}

after #771.

[sourcery-ai]

Hey - I've left some high level feedback:

• The --enable-auto-tool-choice and --tool-call-parser hermes flags are now always enabled for the metal backend; consider making these options configurable or conditional so non-tool-calling use cases are not forced into this behavior.
• If other vllm backends (non-metal) also need tool-calling support, it may be worth aligning the argument construction across backends to avoid inconsistent behavior between runtimes.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `--enable-auto-tool-choice` and `--tool-call-parser hermes` flags are now always enabled for the metal backend; consider making these options configurable or conditional so non-tool-calling use cases are not forced into this behavior.
- If other vllm backends (non-metal) also need tool-calling support, it may be worth aligning the argument construction across backends to avoid inconsistent behavior between runtimes.

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[gemini-code-assist]

Code Review

This pull request enables tool calling support for the vllm-metal backend by adding the necessary command-line arguments. While this fixes the reported issue, the current implementation hardcodes these arguments, which applies them to all models. My review includes a suggestion to make this feature configurable to ensure flexibility and prevent potential issues with models that do not support tool calling.