fix: sandbox permissions

[ilopezluna]

This pull request enhances the backend runner configuration and updates the macOS sandbox policy to better support the vllm-metal backend. The main changes include allowing custom environment variables for backend processes, updating the sandbox to permit necessary network and file operations, and setting a required environment variable for vllm-metal.

[sourcery-ai]

Hey - I've left some high level feedback:

• The new RunnerConfig.Env behavior never allows clearing or fully overriding the inherited environment (an empty but non-nil slice behaves the same as nil); consider either documenting that explicitly or treating a non-nil slice (even if empty) as the complete environment to give callers more control.
• Hard-coding VLLM_HOST_IP=127.0.0.1 in the vllm-metal backend may be too rigid if the bind address ever needs to vary (e.g., different network namespaces or non-localhost setups); it might be safer to derive this from configuration or a helper rather than embedding a literal.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new `RunnerConfig.Env` behavior never allows clearing or fully overriding the inherited environment (an empty but non-nil slice behaves the same as nil); consider either documenting that explicitly or treating a non-nil slice (even if empty) as the complete environment to give callers more control.
- Hard-coding `VLLM_HOST_IP=127.0.0.1` in the vllm-metal backend may be too rigid if the bind address ever needs to vary (e.g., different network namespaces or non-localhost setups); it might be safer to derive this from configuration or a helper rather than embedding a literal.

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[gemini-code-assist]

Code Review

This pull request introduces the ability to pass custom environment variables to backend processes and updates the macOS sandbox profile to allow outbound network connections and additional file access for the vLLM backend. I have no feedback to provide.