fix(e2e): send SIGINT on teardown to prevent orphan processes

[doringeman]

The e2e test teardown was using exec.CommandContext which sends SIGKILL when the context is cancelled. This kills the model-runner server instantly without giving it a chance to shut down its child processes (llama.cpp, vllm-metal), leaving orphan processes running on their ports. Setting Cmd.Cancel to send os.Interrupt instead triggers the same graceful shutdown path as Ctrl+C, which properly cleans up all backend processes.

Note that os.Interrupt is not supported on Windows , so a platform-specific mechanism will be needed when Windows e2e tests are added.

[sourcery-ai]

Hey - I've left some high level feedback:

• Consider making the custom Cancel function resilient to being invoked before the process has started (e.g., checking server.Process != nil before signaling) to avoid potential nil pointer issues if the context is canceled early.
• You may want to add a follow-up mechanism to escalate from os.Interrupt to a Kill after a timeout if the server fails to shut down gracefully, to avoid tests hanging indefinitely on a stuck child process.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider making the custom Cancel function resilient to being invoked before the process has started (e.g., checking server.Process != nil before signaling) to avoid potential nil pointer issues if the context is canceled early.
- You may want to add a follow-up mechanism to escalate from os.Interrupt to a Kill after a timeout if the server fails to shut down gracefully, to avoid tests hanging indefinitely on a stuck child process.

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[gemini-code-assist]

Code Review

This pull request introduces a graceful shutdown mechanism for the server process in e2e tests by setting the server.Cancel function to send an os.Interrupt signal. However, the current implementation will cause issues on Windows, leading to orphan processes, as os.Interrupt is not supported on that platform. The feedback suggests adding a runtime check to use server.Process.Kill() for Windows and os.Interrupt for other operating systems to ensure correct behavior across all platforms.